...Dai Kaioshin WEAKENED him, and is the ONLY Kai that did. This is stated in the dub, so you can't claim ignorance, because you evidently have seen the episode. His good influence and happy demeanor, etc., etc., made him weaker. South Kaioshin, who was close to him in strength, and was ALSO the strongest Kai(this is also stated in the very same episode/volume), but as he is NOT Dai Kaioshin, he did NOT weaken Kid Buu, but instead STRENGTHENED him. South Kaioshin made Kid Buu into Buff Buu, and from what little evidence we have, he's stronger than Super Buu. Dai Kaioshin made Buff Buu into Fat Buu, who is stronger than Kid Buu.
Let's look at it this way(it will work for any numbers, regardless of how you work it). BTW, these are completely random numbers, so just put in anything here that makes sense, and it'll work the same.
Kid Buu: 10
South Kaioshin: 9
Dai Kaioshin: 8(Supposedly he was the next strongest, but I've got no real evidence of this, but it works with any number that is lower than South Kaioshin's).
Kid Buu(10) + South Kaioshin(9) = Buff Buu(19)
Buff Buu(19)+Dai Kaioshin(-8)[Good influence stuff taking effect here, therefore it lowers his power) = Fat Buu(11
By that, it shows that Fat Buu was indeed stronger than Kid Buu, if any doubt remained in your mind, and therefore, Super Buu, and all his transformations are as well, with Mr. Buu being weaker. Evil Buu is debatable on this one...Some people say he's stronger because he absolutely demolished Mr. Buu, but others say that it couldn't work since he's only a part of Fat Buu(though we don't know how much of a part he really is).
Kid Buu: 10