Definitely each to their own, and thank God for that. People are all matched for different sizes. But you should be the healthy size for your own well being (what a cliche, but true) and it doesn't matter what that is.
I've known guys who like their girls so super-thin they make them go on diets (though on these two occasions I remember thinking the guys had control issues generally - what and what not their girlfriend was "allowed" to study, do, talk, say, etc, urgh) and some who speak about girls with curves looking better.
It's when the partner pushes the other person outside comfortable boundaries for that person that it's a worry. The anorexic-thin look is a major worry and also problematic for naturally thin girls who get bullied for being so - high school is big on this, nicknames like "Ana" for chicks thought to be anorexic who are just thin.
There are occasional issues in the other direction, too. Kate Upton, Kelly Brook and Christina Hendricks are all hot, all good weights, and they're all sexy chicks (especially Kate.. straight female here btw, but wow). I saw a doco over a year ago about guys who have fetishes about fattening up their girls, which was disturbing (it was basically psychological abuse) and I thought, "well it's rare enough that I don't see it," and boom! last month I saw it in a couple I know IRL. It explained a few things, not just her growing weight but his controlling/derogatory comments increasing towards his own wife in general social conversation (awkward, awful).
Also, I've noticed a lot of men have major body issues more recently because of men's magazines honing in on the same "is your body good enough?" fears that women's magazines do. It's kind of worrying that body image fears have spread to males. Men with anorexia aren't that uncommon nowadays.
Get fit, be happy, for sure. It's good to be proud of your body. Though something did occur to me the other day driving home... the number of joggers on the side of the road has increased by about 500% in about five years. I keep wondering if I'll drive by a jogging collision. I would have to laugh my arse off, sorry. Real-life physical comedy always gets me going.