Originally posted by Tron
I kind of got the impression that they just underestimated his lack of restraint. I'm pretty sure the last thing they were expecting was that Kid Buu's first act would be wiping out the planet.
Fatty was basically a moderating force, because the fat Kaio-shin was such a good hearted being.
The original Buu (a.k.a.Kidd Buu) without Fat Buu's power should have logically suffered a power drop, but he came across as much more powerful then Super Buu. Look at how Goku did against Fatty, vs how he did against the Original buu.. He couldn't even slow him down!
Odds are, Original Buu kept most of the power, and simply ejected the personality of Fat Buu (And in fact, it was stated Fat Buu already suffered a power drop when Skinny Buu ate him, (transforming into Super Buu..) And without that moderating personality, he stopped restraining his powers...