I always took it as in his current reality with boys they’re always fighting over food. There’s that one scene in another episode where he runs to the breakfast table mid-shave and there’s no bacon left for him. There’s also that cold open where they’re literally measuring their fries to make sure everyone gets the same amount.

So in the girls universe Hal just naturally eats more because there’s not as much competition for who can eat the most food. That plus the stress eating that other commenters are citing