Looking at these pics, I believe that Marilyn Monroe would be considered fat by todays standards.
Regardless of era, it's the unique sets of proportions and balance between all the, um, critical parts. This is true for both men & women - take a 6'2" well shaped (modern day athlete) 20-something cowboy from 1870, clean him up, and he'd be a modern day hunk.
Superficial elements regarding weight are the least concern. That is, anyone can lose weight, work out (a lot), tone up, etc to mirror contemporary standards. But if you don't have the basic building blocks, no amount of cosmetic surgery, clothes, make-up, etc is going to make a difference.
These types of debates crop up in sports all the time. But people who know often state that great players can play in any era. Give them a modern diet, work out regimen, etc, and their natural, base talent will flourish.
Same is true for good looking women - a few weeks ago I came across "Gold Diggers 1933" on TCM. Even though styles have obviously changed, every single chorus girl still had the essential height/hips proportion, bone structure and (heart shaped) face that one (still) needs to be a Rockette.