Okay, I would agree that Isabela's are a little on the large side, but look:
Female Hawke's are an average sort of size.
Excluding the exaggerated part at the very beginning of the demo, Bethany's are also average.
Aveline looks pretty reasonable to me.
And finally:
Not the best picture, but Merill is kinda on the flat-chested side of the spectrum.
Y'know what people say about her? "OMG PEDO BAIT". It's quite sad, really.
Anyway, my point is, not "every single female character" has a massive bosom, and those that do... there are massive bosoms in real life, you know. Aside from that, it's probably not a good idea to judge the cast of Dragon Age 2 based on a short demo.
Don't get me wrong, as a woman, I find it incredibly frustrating when female characters are over sexualised or ridiculously proportioned, but all of these characters look reasonable to me and Isabela is a very sexual character to start with, she plays up to it. At the end of the day I've not got any complaints and to be honest, the models look a heck of a lot better than they did in Dragon Age: Origins. At least there's some variation now, instead of them all looking big-shouldered and identical.