They could stuff pillows under their shirts, perhaps? Or, if a more high-tech solution is warranted, there are those harnesses that actresses wear, when their character's supposed to be pregnant. Something similar could be employed for playing People of Size, generally. It could be filled with birdseed, perhaps, to give it that dimpled, cottage-cheese effect. And, of course, if a 2nd chin was required, a neck bladder could be spirit-gummed to an actor, to complete the effect. CGI could even be used, if allotted for, in the budget ...