The only thing I can think of preventing larger worlds would be complications with split screen or multiplayer? Chunks loading and unloading in different parts of the world depending on where the players are? It can support eight players at a time I believe. I don't think PC needs to worry about that, correct? Since they have servers that take care of that. I really don't know. However, it would be better to be direct instead of giving me, "Yeah, we want to limit your save file size."
No, 4j Steve said that it doesn't have to do with splitscreen.