Discount the prejudices of modern 'fashion', and you are left with genetics, which did well for countless generations;
Women will tend to find larger men attractive, coming from the hunter/gatherer insinct, a larger male shows access to more food, more food means more ability to feed offspring.
Men, tend to be attracted to larger framed women, especially around the hips, this is one of the reasons that corsets became so popular, as it accented the hips.
All in all, I've dated larger women, I've dated stick insects, and you know what, I'd rather have the larger woman that is comfortable in herself, than the stick insect that is constantly dieting, exercising, and never happy about the way they look. Two extremes I know, but such is the way of life. You don't chose who you fall in love with, it just happens.
And those of you who want to go on and on about 'flat stomachs' and the such, just wait until you're into your thirties, and you have a couple of kids, cuz I can guarantee that your partner won't have a flat stomach any longer, or slender hips.