Too big a jump.
If your kids ask for another one (potato, cookie, peanut butter sandwich) I think it's helpful if you just say "Sure!" and make another one, even if you don't think they'll finish it, even if you think they'll be too full or whatever. As long as they're not eating someone else's share (and even so, if the other person agrees), it's not a big deal. If they don't finish, save the leftover for someone else. If they do finish and they're "too full" that's how they'll learn their capacity (which will change anyway as they get older).
photo by Sandra Dodd