In No particular order of importance.
1: Rarity/Status: Like it or not if left with no social discouragement otherwise men will more naturally though not always gravitate to a position where they view women as sexual objects. Part of mens subconscious line of thinking is that a woman is a life defining challenge that must be acquired and secured. With such men put an unusually heavy emphasis on the type of woman the pursue because the woman they get acts as a reflection on them.
So you will find that they will proactively seek a woman who represents uncommon quality. These skinny girls much as has been expressed by people complaining about unfair body image types, are not "realistic depictions of what a woman actually looks like" They hold that uncommon quality, in that they have a physical body that is harder to come by. In short men want what they perceive to be special and different. The ideology of "Why would I want her if anyone else could have her." comes into play. Something to make her special and unique setting her apart from the crowd. So to anyone who sees a girl with a guy one of the first things they will be able to determine about the girl without even speaking to them is her body weight, which instantly conveys the image of "Look, I caught a good one!" to anyone who sees the two of them together.
2: Physicality
Invariably body shape and size comes into play. Some people will just naturally fit together better than others do. I know I speak from first hand experience that being over 6`5 with a fairly low center of gravity that physically it is harder on me in sexual situations to be with a girl in missionary position for example. The reason is, I will end up either having to curl her up like a ball or end up essentially doing a split to get the best angle possible when with a skinny girl with no ass. However my body type is not normal. However with the perfected average 5`11 male body weighing in at the ideal 140-160lb range physically a smaller, skinnier girl would fit a little better.
3a: Inferiority complex
One of the more shallow as well as illogical sentiments I have ever heard against heavier girls is rooted in male insecurities. Just like how women have the whole "bad body image" thing, it is well known men tend to think their penis size is insufficient. So some men will subconsciously think that "every inch fatter she is, the smaller(length) my dick is going to feel to her". It is illogical because even though the optimal depth is only 2-3 inches, men have a tendency to think they need to be deep enough to be "screwing her uterus."
3b: Superiority complex
Much as said before men place disproportionate focus on the size of their junk. Its also true that sexually most men are both visual as well as auditory. Arousal stems predominantly from seeing sex, as well as the sounds of sex. (Conversely this is also why Porn tends to resonate more with men than women who tend to hold sex on emotional and tactile levels) So even if it is an incorrect assessment, there is a rationale of proportional body size. I have heard in the past "A bigger girl is automatically going to be more loose" The line of thinking here is that if a woman is taller or wider, then it will somehow make the vaginal canal slightly wider. So by that flawed line of reasoning, A wide girl is wider, a thin girl must be thinner. The desire for a narrow set vagina is obviously for tactile sensation, but it is also centered around the females physical response. Men typically want to "hear" her reaction. The moans and sounds she will make in response to his touch. So coupled with the irrational fear of an inferior penis size, it makes sense that her smaller size will somehow equate into his smaller size not being as impactful.
4: Women bring it on themselves
DaMullet said:
Women are killing themselves and falling into ruts of depression over this.
Despite what you may think about men, it is really not men (as an entire gender) who try to proliferate the negatively reinforced body image that women experience. It centers around advertising and marketing more than a reflection of male desires. It has long been documented and studied that women respond more to advertising than males. This is why advertising and marketing that we see, hear, read, etc is 80% or more geared toward women. With that said it is also well documented that fear and inferiority make for very effective marketing strategies. So yes you will see ad execs intentionally push negative reinforcement and perpetuate inferiority complexes because they want to project that "THEY" are selling the "cure". "You too can be beautiful... Just buy this!" So this is an area that the blame is not for men as a gender (perhaps specific male ad execs) and it is not even an issue of feminine insecurity or lack of self confidence as much as it is that as far as it comes to advertising, women on average more susceptible to advertising. Whats worse is that not all women are that susceptible, but the marketers know that the ones who are, will invariably act as a carrier and infect those who are not through social interaction.
5: Dominion
This has a tendency to fall under gender defined roles. Its an almost natural state (though obviously not a rule) for in heterosexual relationships for the male to be sexually dominant and the female to be more submissive. Its not really clear if that centers more around mens desire to feel in control, or if it falls to various other factors.
Typically men are physically stronger than women (Again, not a rule) So inherently there is a degree of dominion already bestowed. However the larger the gap in physical strength between the male and female, the larger the sensation of dominion grows. So when a male is with a female closer to his own size and strength that sense of dominion becomes greatly diminished because its harder to dominate someone your own size. So to an effect some men will favor someone who projects a sense of being weaker and more delicate.
Physically during sexual engagement it is also much harder to hold the entire body weight of a girl weighing 170lbs than one weighing 120lbs. Thus it requires a greater expense of energy and stamina to do so. Thus it can create a sexually limiting factor to those who play rough and take the roleplay of dominion further than the average will.
6: Virility and fertility
It boils down to genetic precursors that create subconscious desires for sex to be productive and thus reproductive. It has been said that Women favor experience and ability, men favor youth and fertility. Both work in conjunction to create the ideal conditions for the possibility of procreation.
Humans invariably increase in body size as they age until they plateau and begin to wither into old age. So from a physical perspective a thinner girl will project an sense of youth and fertility than an older woman will be able to.
As a side note on this matter, It was suggested that the root of this is a naturally latent desire in men towards Hebephilia and ephebophilia, and pursuing a mate who is smaller allows one to create the illusion of someone who is under age. I disagree with the assertion, because it implies a criminal intent, whereas conceptually the age really has no impact on the decision when compared to the prospective fertility, and the latent desire to produce healthy offspring. Chronophilia has its own set of terms and conditions for its presence and there is little or nothing that suggest a correlation of factors linking the two.
7: Health
While tied into fertility, body weight has impacts on an individuals health and stamina. While not impossible (as the prior poster referenced ), it is rare that you see sumo wrestlers running marathons. It stands to reason that someone who weighs more will expend more stamina to accomplish the same physical intensive task as someone who is lighter. So if less energy is expended, the perception may be that it will allow for longer sexual sessions and include more of her physical exertion in the act. There does seem to be a resistance to women who "Just lay there and look pretty"
8: Status and perception
There are portions of this that fall under the way people present themselves and the way they are perceived. If you compare two similar females one being slightly under weight and one being slightly overweight, based on a glance that impression might suggest that the smaller girl either holds greater status on her own (such as money to support better diet and health care maint such as exercise equipment). as well as a suggestion of someone who has their life together and is able to maintain a regulated existence that balances a diet and takes the time for exercise, while balancing the other parts of their life. That can be an alluring factor for some people.
Oddly, it can also represent the exact opposite, but still remain viewed to be a positive factor. If a girl is underweight to the point that it suggests that she might be impoverished it might also suggest that she would be easier to approach because she might be open to someone who could offer her opportunity to take care of her better than what she currently experiences.
