How to talk to girlfriend about her eating?

I'm in a fairly long term relationship with this girl, she had slowly started gaining some weight, nothing too much. But recently she has started to really look fatter, she says about how fat she is getting, yet eats tons of junk all day. We went out for dinner the other week and she had a big lunch a couple hours before, and a huge dinner she could barely walk she was so full, but when we were home an hour later she unbuttoned her jeans and was just eating crap. I'm worried she is going to get too unhealthy, I'm a chubby girl but she is getting huge, always unbuttons her trousers at home now.
How to talk to girlfriend about her eating?
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