As TV Tropes explained:
"Someone is preparing to take his share of a large item (a pie, money, or as in the page image, a pizza) that is clearly meant to be split between several persons. He cuts a small portion out from the main lump — and then leaves that behind while he makes off with the rest. The tinier the initial portion, the funnier the joke becomes: what appeared at first to be moderation and restraint on someone's part is actually just greed, since a smaller remnant means more for him."

All though Treecko didn't cut the slice himself, Grovyle cut a slice, and Treecko ends up eating the rest of the big cake. Now theres a clear difference between the bigger and the little brother.. pfft