Does it seem to you like, in about every character Michael J Fox plays, he always has to make at least one smartass comment? Even in his minor roles, he manages to throw out a zinger.

Marlene: Oh, who's going to eat all that?
Marty Jr: Grandma, can you just shove it in my mouth?
Seamus: I've never had any complaints about it from the pigs.

Ironically enough, even though teen Marty has made quite a few smartass comments throughout the trilogy - his older incarnation from 2015 doesn't. By then, he's so stodgy and uptight - and he chastises his son for being the same way he used to be.

Marlene: Oh, who's going to eat all that?
George: Oh, I will!!

I always thought Marlene was being a little sarcastic with that comment, but some people thought she seriously was wondering. Of course, the joke is supposed to be that the pizza looks incredibly small - which is the out-of-universe explanation. Even in it's full size, though, I don't think there was enough pizza to go around. There were six people, and eight slices - and I don't think anyone can eat just one slice, unless there's other food to go with it.