It's just an opposite reaction to the anorexic models and whatnot. Two extremes of dumb advice. That said, my personal taste in women is a bit on the curvy side, but not obese. I'm assuming many guys are like that, and yeah.
I think what it stems from is partially, BMI and scale numbers are very inaccurate in the realm of determining your actual healthiness. Like take a lot of female athletes, according to BMI they've overweight, many hovering in the 25-26 range. It's because BMI doesn't take into account muscle mass and bone structure. Like for me, to even hit 25 BMI I'd have to be sub 10% bodyfat. So for me to use weight recommendations and BMI, I'd be spinning my wheels at best, and at worse, it'd be unhealthy. There's also the classic ectomorph, mesomorph, and endomorph subtypes, and while those aren't like totally accurate, there's obviously people you see that fit those types, and it's just their bodytype and there's nothing they can do. So there is a grain of truth in these new kinda campaign things, but the problem is, it's telling people, say, 150lbs is OK to be no matter what your bodytype and height, rather than 150lbs being good for a specific bodytype/height/activity level.
I think some of it, too, there's simply less skinny people now, due to the food supply being so crappy and nobody cooking things themselves anymore. Now fat people are the majority really, so it's much harder to make fun of fat people if you are one. So now, the bar is lowered and what constitutes as "skinny" now could be 20-30 years ago's "fat."
Another thing you gotta consider, too, the media representations of stuff. I kinda have a theory that back in, say, the 40s or so, you were kinda only "fat" if you were grossly overweight, so a "little extra" was sorta OK, but then once movies and media became widespread, and for all intensive purposes, everything got skimpier for both sexes. So now people see ideal looking people for both sexes all over everyday, not just once in a blue moon, so now "real people" are judged against people basically paid to look good.
Yeah, it's all dumb and reactionary. It's pretty much just due to people being dumb. People just need to concentrate on their own body, and not compare it to others, but at the same time, not comparing to others doesn't mean being lazy and unhealthy.