<<sigh>> Filler strikes again with its contracting lines and BS.
Right here goes I'll try and explain this, though this may go horribly wrong lol.
Right the order of power goes like this
Gohan Buu> Gotenks Buu> Piccolo Buu> Buff Buu> Super Buu> Fat Buu> Kid Buu=Evil Buu >Mr Buu
Right then Kid Buu was the weakest but the most deadly as he had no pure souls holding him back and he would destroy anything without hesitation, except maybe to prolong suffering. He then Absorbed the South Kai who turned him into Buff Buu, This was who Goku was referring to when he said he was getting stronger after he and Vegeta freed everyone inside Buu's Body. After this form disappeared and he reverted to Kid Buu he and Vegeta commented on how they could now take him. Anyway after he absorbed the fat Kai and became fat Buu he became stronger but lost his killer instinct. He then separated into Mr Buu and evil Buu (Mr Buu looks exactly the same as fat buu) and evil buu ate fat buu to become Super Buu adding their power levels together, the rest is then self explanatory really as everyone who Buu absorbs gets their power levels added to his.
And before it comes up yes I know that Goku at Super Saiyan 3 said he could beat Fat Buu but couldn't finish kid buu, but after spending years on DBZ forums this is how it was explained, When Goku was dead he could fully power up as a super Saiyan 3 but when alive it drained to much of his chi which he did state and he couldn't get up to full power, which is why he couldn't defeat Kid buu.
And as for the Buu being a threat to the entire universe because he was punching loads of holes in dimensions against Vegerot (or Vegito as he is known outside the manga in case people didn't know) that was filler, yes Buu can create holes but as Super buu he only managed to make 1 small hole for him to just walk through, hardly a threat really. And when they say he was a threat to the universe Its simply because he can go anywhere he wants and besides a select few of the Z warriors No one can even slow him down. So he can destroy anything he wants to with no resistance not blow up the universe in one shot (Unless your all saying that the entire human race's chi combined could do it because that's what formed the Genki Dama and killed kid Buu)
1 More thing some people can get around the whole Candy thing, Vegerot was turned into candy and he still kicked the crap out of Buu...
I hope this cleared up anything though I'm sure someone will have a go at me for this...oh and sorry if it sounded condescending anywhere in there.