Dax
Doubtful, as that would involve modifying the model itself, which can't be done without requiring everyone to install mods.

Or maybe they can do it with resource packs, I don't know.
3 comments
Well, they can do it with the wither just with bukkit (It's also possible in vanilla with command blocks.)
Dax
That's because the wither already has a smaller form model ingame, for when you're spawning one. They can just prevent it from growing.
I'm pretty sure you can spawn them in various sizes, just like slimes.