Of course, we did notice. I think Billy might have an eating disorder of some kind (I have one myself and it sucks), actually I think I remember him alluding to it at some point, referring to "Siamese Dream"(I wish I could remember and find the quote/source). A way for him to deal with pressure and anxiety ? This could be totally wrong and sound completely stupid but take a look at the Metro 93 DVD, or at the MTV Europe acoustic set from 93. Billy's by no mean fat, he's just a little soft and his face is a bit fuller, you know. Just coming out of months of writing and recording SD, which was a very tough period personally and a low point in the band's career. Billy looked a lot slimmer during the 94 Lollapalooza shows ("The lost 94 tapes" for instance), of course, sweating it off on stage does help.
Same for "Mellon Collie" - he looks a bit fuller in the "666" tapes than a few months prior and by 96, he's "back to normal". Some people could say "sitting on your ass at home writing songs, of course you're gonna put on weight" but in Billy's case it's always quite fast, don't you think? In 2008 he was quite scary, honestly (but in rock n'roll, it's fine to be emaciated, hardly noticed, but you can't be "fat". What the hell...).