The thing with guys like Freiza and Cell is that they're final forms, while smaller than previous ones, were more humanoid and "sleeker". The previous forms are big and monstrous.
This isn't really the case with Mewtwo. The original form isn't some big buff ugly one like with Cell or Frieza's previous forms. In fact I'd argue the new form looks more foreign and alien than the original.
I suppose the idea may be bigger head = more psychic power...?
"Combine Cloak and Dagger with Boots of Swiftness so CC doesn't stop you from moving faster toward defeat." - Frost_shock_FTW
Kid Buu was indeed the original form, but it was still his strongest form. Evidently the goodness in fat Buu (originating from those Supreme Kais he consumed) took some edge off his power. Getting rid of that made him pure evil which somehow made him stronger/unhindered. You obviously didn't pay attention to all the details.
Kid Buu was the most dangerous, but not most powerful.
There is nothing that can prove it wasn't both. People assume that since SSJ3 Goku held his own against Kid Buu, Super Buu must have been stronger. Villains held back the lion's share of their power throughout the series. Nothing can prove that he was any weaker than Super Buu
"I am the voice of one among the silent who's tired of burning among the flames"