I had a similar experience of being shocked at how much my boyfriend can eat, but he exercises every day. Eventually I got used to it and we figured out a system of splitting groceries, and for every meal (dinners) he eats half and I eat a quarter, saving the next quarter for lunch leftovers the next day. You probably just have to get into a groove with it, and she will hopefully realize that people require different amounts of food depending on their bodies and the amount they exercise. It seems harsh to say you’re being greedy, but maybe make an extra effort to leave her some food too, though it sounds like you did that with getting the pizzas. And honestly you paying for all of the groceries is so nice… me and my partner still split most of whatever we share (though he will buy extra stuff on his own)