The size the summon appears on your screen is already client side. All the server is doing is keeping track of its position, actions, and such. If the game is moddable at all, this is probably easy to do.

But more importantly, summons have an in-game, lore reason for being what they are. Whether that was done for technical reasons at first or not, it wouldn't make sense to change it now. Local mods are probably the best solution for people who want larger summons, but I wouldn't expect them to change the game itself.