The only expression I can think of is one I learnt off my Canadian wife. 'I feel like the pillsbury doughboy' or 'I'm going to end up like the pillsbury doughboy' or 'You're going to end up like the Pillsbury doughboy'. ALWAYS said jokingly and in light humour after a big meal or when we've eaten too much. You'd never say it to anyone other than a partner, otherwise it is very insulting. The term comes from a mascot created for the Pillsbury Company ages ago. He was fat, made of dough and used to jump out of a can of refrigerated bread dough. A lot may not know the term now ? And anyone outside of North America probably hasn't come across it.

Nothing. Treat her as a person, and not as a weight condition. Teasing is frequently more enjoyed by the teaser than the recipient, and is sometimes little short of bullying. Find another way to play with her, may I respectfully suggest :)