The internet exposes us to astronomically more people than we ever could have seen in the 80s and 90s. Really heavy people are still rare today, but if you look at enough pictures of enough people, especially if these sites showing these pictures self-select for oddities, you're going to see quite a few, which makes them seem much more abundant today than they really are, plus much more rare in the past than they really were.

Again, look at pictures of groups in candid settings and you'll notice similar levels of obesity.