What the thread title said. Let's ignore power level arguments about how much stronger than Kid Buu Super Buu is for the moment. I've been thinking a bit, and I think I kinda figured out one of the greatest mind screws of the series: why Super Buu reverts to Kid Buu after fatso is ripped out, even though Super Buu himself is just Evil Buu with fatso absorbed.
The premise is basically this: the Good Buu that Evil Buu absorbed and the Good Buu that Vegeta ripped out (who shall now be referred to as Mr.Buu to differentiate him from the previous Buu) are different, due to an automatic restructuring of Buu's body after Super Buu was formed.
When Fat Buu got angrier than he's ever gotten, his power spiked due to accessing his hidden ki and his anger swelled within him. His evil ki took a new form, the being we know as Evil Buu. Now, even though this being is pure evil, the fact that it came out of a Kaioshin dominated being means it has Kaioshin influence and probably even Kaioshin ki in it, albeit unusable ki. It's clothing and sickly appearance if anything implies that the Kaioshin influence inside is detrimental, as the Dai Kaioshin had a weakening effect on Buu when he was absorbed.
This piece of the Kaioshin influence and power is what makes Evil Buu and Kid Buu different; Kid Buu really ceased to exist after Fat Buu formed, as Dai Kaioshin and Pure Buu couldn't exist inside the same being. However, his essence and power were still there, falling under the umbrella of Fat Buu's evil. This essence and power was scrounged up along with said Kaioshin ki and influence (I will now just refer to that as "Evil Buu's impurities") to create Evil Buu. Evil Buu, unlike Kid Buu, could actually talk and recognize the gains of becoming whole, further showing his Kaioshin influence, which gave him more intellect (embodied in Super Buu). When Evil Buu and Good Buu united, the evil being dominate essentially resulted in Kid Buu being reborn, and becoming the base of Super Buu (just look at them; they look the same except Super is taller and more muscular). What happened next was essentially an automatic purification and rearrangement of Buu's body.
Which brings us to Mr.Buu. Due to the above rebirth, all of the impurities that diluted Kid Buu's essence within Evil Buu were taken out and placed in a new convenient package- Mr.Buu. This made Mr.Buu stronger than the earlier Good Buu. These impurities, unlike the rest of the Kaioshin power, couldn't be used by Super Buu (evidenced by Buff Buu being stronger than Super), but they could be used by Mr.Buu (post-split). Basically the second split between the Buus was the more perfect split, as both Buus were at their maximum possible powers and not missing pieces.
Evil Buu = Kid Buu + impurities (may be unusable and/or detrimental)
Good Buu = Rest of Kaioshin ki/essence
Super Buu = Kid Buu + Good Buu (most of Kaioshin power) + Evil Buu's impurities (unusable power)
Kid Buu = Evil Buu - impurities
Mr. Buu = Good Buu + Evil Buu's impurities
In other words, Mr. Buu = South Kaioshin + Dai Kaioshin. As in, their power and their essence, not the Kaioshin themselves; they've long since ceased to exist, and Mr. Buu is his own being.
This, among other things, explains why Super Buu didn't turn into Kid Buu, and why Mr.Buu was able to land hits and put up a fight against Kid Buu when he got annihilated by Evil Buu. Though from a strictly power level perspective it requires you to believe that Super Buu isn't multi-folds above Kid Buu- but that's not the point of this. Just from a basic logical perspective, how does this theory sound? What is your own theory?