I wonder how much of it is studio mandated and how much is due to the vanity of the actors. Damon, for instance, would have felt a certain amount of pressure to look good for the last Bourne film (which should never have been made, by the way) but I can't imagine any studio execs ever went to him and said "you need to put on X lbs of muscle for this role, Matt.'
Which is another disparity, because I can't imagine that many actresses have the star power to choose to look a certain way in movies. Unless you're Charlize Theron in Monster you're probably not going to win a role unless you weigh less than 120lbs. and in the cases of both men trying to gain muscle and women trying to lose weight, it's really not good for their long term health.
So we're going to see the very odd juxtaposition of Chris Hemsworth's gargantuan arms justifying his superhuman strength in Thor 4 (More Thor) and Natalie Portman having the same superhuman strength while being whippet-thin. If the strength is superhuman, why does he need the muscles? And if he needs the muscles, why won't Portman?
I've always thought that the superhero body type should be the same for men and women, at least when it comes to how they're drawn in comic books - they should look like heptathletes and decathletes. Multi-discipline athletes with excellent functional muscles but not ridiculously huge (except for the likes of the Hulk, of course).
I'm not an advocate of plastic surgery, and often can't really tell who's had it and who hasn't, but I do know of one case where it has to have been transformative. Stana Katic had a nose job and went from pretty-but-normal to stunning. She never would have been the co-lead on Castle without it. I'm sure that was her choice, and it paid off.
Other women will surely have been told 'change this, work on that' if they want success. Similar to the code that wrestling promoters used to use - "work on your upper body" pretty much meant "get bigger, even if you have to use steroids."
The anorexic look is one I just never got at all. It did not look good. Calista Flockhart and Portia de Rossi both looked like they were sick, as did many other women.