Pokémon X and Pokémon Y

''Smaller'' can mean more potent depending on what is being condensed. ''Energies'' are usually far more powerful when condensed. If you've ever seen characters in cartoons and crap that have some sort of energy-charge attack they usually show the charge getting smaller as time passes. This makes it more powerful. Works in real life too of course. I think Buu is also based off this logic.

Like said already, Mewtwo is a special (energy) based Pokemon. Doesn't even really matter though since it's a game, and a game about magic critters with powers so... You have crap that can transform into anything, so why not just get smaller?