For one thing, if someone like Twisted Fate is already gigantic, what will happen when you remake someone like Cho'Gath?
Scale inconsistencies only look truly odd or broken when you're dealing within a given group or species, especially if that group exists in the real world.
For example if we decide to make a heroic human male character we have to take into account that everyone knows what real world humans look like. You go in with some set of guidelines. You can bend and push those guidelines pretty far, but there is a limit within any art style. All of our characters are pretty stylized but if we make other humans then they ought to at least relate somewhat to one another.
Now with a fantasy species that doesn't exist in the real world like Cho'Gath there isn't a reason to change his scale. Exactly how big is a Cho'Gath??.. depends on how fed he is, that's about it