How do I politely turn away large portions of food?

So I'm trying to lose weight. But it doesn't start there, my girlfriend doesn't like the idea of me losing weight or going on a diet, so I'm trying to just be happily toning it down slowly. I'm to the point where when I buy two dinners from the freezer section it fills me up completely, which is a lot of progress because I would usually get 3 things, we would eat two totinos pizzas at once seperately, and now I'm to the point where 1 pizza fills me up, and I would be happy with only half a pizza. She is really sensitive about that kind of thing, but I would like to start eating more natural foods with a steak or something here and there. So what are some ways where I can just say "no thanks" without using those exact words? I want to get healthier. And hopefully as a result thinner.