Who cares about why. Complaining about it solves nothing. There are men that find bigger girls attractive, it's just going to take extra work to find them. In the long run, working out and exercising is a good idea for everyone. Mainly because it makes us more sexually appealing. I'm scrawny as hell, but I'm damn charming. Alas, charm only goes so far. It's taken me 2 months to go from a 75 bench press to 100 Lbs. I want f***ing pecs dammit. It's probably going to take me another 6 months of dedication to reach 200 Lbs.
I could ask the same thing about girls?... Were all human and were not as different as some like you tend to think. women are probably even more particualr about types and ideal men. Plus your wrong about the perfectly thin and pretty part. if you watched the last season of the bachelor one of the last 4 women had a fair amount of meat on her yet was very attractive. And another had a masculine look to her face. Goes to show you were not like how you think we are.
Magazines,tv shows , models basically everything has the stereotypical " normal girl " the girl that is perfectly thin , toned ect. So now all guys have this level that they expect all girls to level up to unless there " unattractive. "