The world's population, particularly in Euroe and America, grows heavier every year. Despite governments' attempts to stop this, people become bigger. Regardless of what we think about this personally, all these people will have to be dressed, and I assume most of them would like to dress nicely. So basically fashion will have to change to include larger people, simply because that's how the market works.
Unfortunately I believe heavier models will have a difficult time fitting in the "best" shows for a few years yet, because the transition is very slow.