My girlfriend has gotten bigger and I don’t know how to bring it up.

Don’t tiptoe around it. There’s absolutely no reason we should kowtow to the Fat Acceptance movement clowns who say it’s wrong to lose attraction to a partner because they start ballooning to the size of Arcturus.

Attraction (contra Gen Z and modern feminism) is natural and so is losing it when someone becomes something through their own will (or lack of it) that you did not sign up for. That women should never be criticized for anything related to their appearance when in a relationship is also insane. Men are afforded no such grace.

So, talk to her about it. If she reacts badly, end it. That’s really all you can do.