Let me give you a personal example on how 1) eating all her food doesn't mean she's less fit or that she's a pig, and 2) a simple joke can make cause eating/self-esteem problems. I'm 5 foot 3 and 120 pounds about. I never work out, I barely walk (just at college), and I typically finish all my food at restaurants, unless it's bad food. I bet if you saw me, you wouldn't call me a pig because I don't have a 'few extra pounds.' When I read how active your 'girlfriend' is, I thought: That's great, she must really work up an appetite. And that's probably the truth, man.
Also, about number two, my mom used to make fun of what I ate and would tease me about my weight. I soon after became anorexic, and soon to follow, bulimic, and then to follow, binge eating. All from simple comments. Your 'girlfriend' most likely already has self-esteem problems, so ..."if you can't say something nice about her, don't say anything at all."
If you didn't want her eating her food, then don't invite her to a restaurant. Most women don't eat on dates, but I bet when you marry them, they'll eat just like 'a pig'. PS: Eating a GOOD thing.
