We often think it's the other way around, that we try to make our bodies fit the clothes, but maybe not so much nowadays.
Take for example the high rise waist on trousers and jeans, voluminous dresses, elasticated waistbands even in younger shops. When I was a teen I could never have imagined chunky elasticated waists would become so mainstream.
Before this it was leggings, joggers, what we all call 'comfy' clothes that allow us to expand.
A lot of us prefer looser fit clothes no matter what our bodytype of course. But the fact that the entire clothing market, at least for casual stuff, has grown larger and more accommodating in recent years does make me think I'm possibly correct.
And it's great that we have these options, and it saves having to buy new stuff when we gain a few pounds, I ain't knocking it, but I do think the market has expanded these styles to fit a growing average BMI. When you see threads about high waist jeans people defend them often because (and I quote) "they suck all my flabby bits in" 
I am adding myself to that, so no digs from me!