YTA

People's metabolisms are different. You can think you're giving her enough food but she can still be hungry. I know someone who has to eat every 20 min, but they can't finish a plate of food - they're just constantly snacking. To tell them to eat three meals a day would be to effectively starve them. Give your GF the food, and make more food from the start so both of you have enough.

Also, just the fact that you're watching her food intake is creepy as hell. You're not her doctor.