For one reason or another, Zero is very bent on stopping Kirby, but doesn't use Possession to do so. I can't say why because we don't know exactly why, but that's how it is. And whether you want to call it a corruption or not, my intention was just to indicate that game mechanics are worth taking into account, especially because even Zero's "larger than a cloud" size is way bigger than the 2 meters you fight him at in-game. If I didn't make that clearer, I apologize.
Also you can't assume he changed his size between his arrival on Popstar and his fight with Kirby because he was never shown doing so.
Well, if you're very bent on using strictly physics, total energy isn't solely composed of kinetic energy. Yes, I should've mentioned this sooner, but there are far more types of energy. To begin with, potential energy is another type of mechanical energy alongside kinetic energy, and there are subsets of both potential energy and kinetic energy. On a broader scale, mechanical energy is only one of six types of energy. The law of conservation of energy dictates that energy cannot be created or destroyed, only converted into one form or another. If you want to fully apply physics, you must also acknowledge that Zero cannot be less than 5-A without violating the law of conservation of energy.
Well I've given a new argument, and I'll also take this time to apologize for getting heated. Even if I didn't directly insult you, I was getting very aggressive.