SRP76

The "Buff Buu" that we see in the manga isn't exactly the one that was formed when Buu absorbed the Kai. He's much, much stronger.

This one is based on Super Buu's power level, not Kid Buu's. It's subtracting Fat Buu from Super Buu one component at a time, not all at once (if it happened all at once, we'd see NO "Buff Buu" at all, since the Kai is entirely contained within Fat Buu).

You have three parts of Fat Buu:

1. Daikaioh (which supresses the power of a Buu)
2. South Kai (which raises the max power of a Buu)
3. the Buu portion itself (there has to be something of Buu to him, or he'd just be two Kais stuck together).

You start with Super Buu, who is >>> Kid Buu.

Step one was subtracting Daikaioh. Taking away the component that supresses Buu's power will RAISE Super Buu's power level. That's what we saw happen. It's Super Buu's power plus more, not Kid Buu's power plus more.

Step two was subtracting South Kai. This started weakening Super Buu.

Step three was subtracting the remainder of the absorbed Buu. This is what weakened him all the way back down to Kid Buu, since he no longer had that huge "Buu fused with another Buu" increase.

Simply having Kid Buu absorb South Kai would get the "real Buff Buu" nowhere near the power of the one we saw, since he doesn't have that boost of another Buu in him.