this poll is actually quite hilarious.. I even voted, even though I don't normally vote on horrible polls, just to see how many idiots had voted.
Swords that were used by real men who wanted to cut and disembowel one another were generally light enough to be used to block other weapons, and find opportune places to strike.
Steel isn't really heavier then iron.. So look at an sword shaped piece of iron.. about 2 feet long, half-quarter inch wide.. It'd weigh about 6-7 lbs. Turning it into a sword would probably make it LIGHTER.. any sword that was over 15lbs would be ridiculously large, unwieldy, and would tire out the wielder after only a few, largely uncontrollable swings. A "Buster Sword" like Clouds would be infeasible for him to wield, even if he had the muscle mass of Barret. There are certain biomechanical limits to what is feasible. I'll agree that a Buster Sword, made of solid iron/steel would weigh a fucking ton, but any real swordsmith would look at it like a joke "Yeah, you're going to use that 50lb monster.. I'm going to use this 3lb.. I'm not particularly scared of you.. I could cut/stab you probably 6-7 times in the time it would take for you to lift your sword off the ground."
So real swords were lighter then 15 lbs, and all the other multitude of answers are just hilarious jokes.