The paper comes to the conclusion that men eat more in the company of women due to sexual selection.

I was wondering if maybe men ate more in the company of women because women eat less, and there was more left over for the man.

But the experiment seems to control for this, as the restaurant serves both salad and pizza in an all-you-can-eat buffet style (probably a Cici's?). So there is, in effect, an unlimited amount of food to eat, but the diners can choose exactly how much they want to eat, at slice-level (for pizza) or bowl-level (for salad) granularity.

As opposed to another all-you-can-eat pizza place that might deliver whole pizzas to the table, which two male friends might be more apt to split evenly, whereas the male in a mixed couple might have more pizza available to him.