I came up with something.
IMO Buu found a way to absorb people without losing strenght.
Fat Buu contained Kid Buu and a few Grand Kais if I remember well. Those few affected Buu's appearance, strenght and mind.
But then Fat Buu got really angry and that was the way for his evil side (Kid Buu) to come out. So Kid Buu got out with a form of evil of Fat Buu and took the most strenght from him. Then, after Evil Buu defeated Good Buu (it's Good one from now on), he absorbed him (by making him into candy if I remember well), but only obtained his strenght and a new form of Buu, that contained Kid Buu and the Kais, but using Kais' power as an advantage. This way wasn't perfect either since Kais have still affected some of Super Buu's mind (he didn't kill Mr. Satan, etc.) but he was no longer tamed.
Now, Kid Buu is the form without the Kais. I'm not sure how strong those Kais were, but IMO Super Buu is Kid Buu + Kais' (whose he absorbed) power.