I’m not going to get into the debate about the rights and wrongs of fat-suits except to say that many times a fat-suit is used as an offensive sight-gag or for offensive stereotyping. On the other hand if a character changes size (eg dramatic weight lose through illness) then it is either going to need two actresses to portray a single character or an expensive makeup job. Yes, sometimes like Renee Zellweger (Bridget Jones’s Diary) or Toni Collette (Muriel’s Wedding) the actresses can change their body shape for real but there are also very good reasons why most don’t: if her schedule doesn’t allow it; if the change is massive; if the character has to be seen at different sizes; if the actress is (understandably) unwilling to gain weight she’s probably struggled to lose to get other parts.
Here you’ll see some of the effects that were achieved with prosthetic makeup as well as fat-suits. For the fat scenes they often wear clothes with long sleeves and higher necklines to minimise the prosthetics: but why oh why is it that so often the characters have to lose all dress sense and the ability to use makeup when they put on weight?