When Fat Buu expelled Evil Buu, wouldn't Evil Buu essentially be Kid Buu, but just look different? And would Good Buu basically be that Kai (the one that made Kid Buu fat) covered in pink buu goo? But do you think that the Evil skinny Buu is stronger then Kid Buu?
The evil buu comes out looking old, tall, and extremely skinny. When he split he very well could have given him other people (along with other attributes) to the the evil buu. Something that is in kid buu has to be still contained with in the good buu after the evil buu was expelled because the pink buu took no transformation after kicking the grey one out! He kept the original power of the kid buu, but still had the kai to counter act it. The grey one just got some other people, but didn't have the kai to contain the power anymore! Thus making the grey one stronger, but not being kid buu........ok..if that made any sense Hooray! If not...your brain probably hurts as bad as mine does!