Im not entirely sure, but I think it's related to how much data was added or removed. That's why it's so big when the page is first made, and really small for typo fixes and whatnot. Again, I'm not sure.
More accurately, the numbers represent the number of bytes added or removed from the page in that particular edit. If you move your cursor over it, a pop-up will mention how many bytes the page has after performing the edit.