Why did Super Boo reverted to Kid Boo instead of Grey Skinny Boo after Good Boo was torn out from inside?

[Kokonoe]

Fat Boo is considerably stronger than Pure/"Kid" Boo. The absorption of Dai Kaioshin merely relaxed Boo's full-power, and made it more difficult for him to access the power of the Kaioshins. He was just toying with Goku during their short spar- and even suppressed, he was about equal to Goku, as the Daizenshuu notes. The only time he ever showed his full-power was when he was splitting into Pure Evil/"Gray" Boo and Mr. Boo, where he temporarily went full-rage. That's why Piccolo's initial assessment of Super Boo only mentions Boo's ki changing to pure evil and his body becoming toned, as opposed to his power increasing - it's only when Super Boo goes to the lookout that Piccolo remarks him to have become more powerful in every way.

Additionally: Boo definitely powered-up during his flight to the Lookout, because when questioned about Gotenks's chances, Piccolo initially only states that they might not be able to win ("that'd be nice, but...[I'm not sure if they can win]), but when Boo pops up right next to them, Piccolo states they DEFINITELY could not win.

Also, Piccolo is the only one who ever sensed Fat Boo's full-power, as shown in the final few chapters, where it's revealed that Goku, Gohan and the others did not see Fat Boo split in two. This is why Goku believes himself capable of beating Boo and wants to revert Super Boo to Fat Boo.

Fat Boo (Full Throttle) > SSJ Gotenks (Pre-RoSaT) > Pure Boo >= SSJ3 Goku >= Fat Boo (Suppressed)

You are just twisting the narrative at this point, its outright stated that absorptions weakened Pure Boo

Chapter: 508 (DBZ 314), P4.2-7
Kaioshin: “In m…my era there were five Kaioshins…until they were defeated by the Majin Boo that the wizard Babidi created…I was the youngest and most powerless one, but I somehow survived, with only heavy injuries…But the other four fell to Boo…First, two were killed…the North Kaioshin and the West Kaioshin….And then the South Kaioshin, the burliest and strongest one, was…somehow absorbed by Boo….. “
Elder Kaioshin: …And he became that huge Boo from before? “
Kaioshin: “…Yes…Next to be absorbed was the fat but kind and gentle Dai-Kaioshin….The Majin Boo that Bibidi created was evil itself, a failed creation that even Bibidi himself couldn’t handle, but by absorbing the Dai-Kaioshin, he somehow become controllable…. “
Elder Kaioshin: “…So he was finally completed…Which is to say…that this current…small Majin Boo…is the initial…mo…most troublesome one…”
Kaioshin: “….Yes…the heart which he gained by going so far as to lower his power through absorption[s]…has returned once again to the way it was…he has no self-control whatsoever…he has become evil itself…”

This is very easily contradicted by the fact that when he absorbed Gotenks and Gohan he became vastly stronger.

I think people are misreading what's going on here. Kai is saying his mentality is less sinister and thus is weaker in terms of that. With Kid Buu he no longer has any restraint on his malice and he'll do anything he wants without care, thus that makes him stronger in a sense.

[Ripper 30]

Fat Boo is considerably stronger than Pure/"Kid" Boo. The absorption of Dai Kaioshin merely relaxed Boo's full-power, and made it more difficult for him to access the power of the Kaioshins. He was just toying with Goku during their short spar- and even suppressed, he was about equal to Goku, as the Daizenshuu notes. The only time he ever showed his full-power was when he was splitting into Pure Evil/"Gray" Boo and Mr. Boo, where he temporarily went full-rage. That's why Piccolo's initial assessment of Super Boo only mentions Boo's ki changing to pure evil and his body becoming toned, as opposed to his power increasing - it's only when Super Boo goes to the lookout that Piccolo remarks him to have become more powerful in every way.

Additionally: Boo definitely powered-up during his flight to the Lookout, because when questioned about Gotenks's chances, Piccolo initially only states that they might not be able to win ("that'd be nice, but...[I'm not sure if they can win]), but when Boo pops up right next to them, Piccolo states they DEFINITELY could not win.

Also, Piccolo is the only one who ever sensed Fat Boo's full-power, as shown in the final few chapters, where it's revealed that Goku, Gohan and the others did not see Fat Boo split in two. This is why Goku believes himself capable of beating Boo and wants to revert Super Boo to Fat Boo.

Fat Boo (Full Throttle) > SSJ Gotenks (Pre-RoSaT) > Pure Boo >= SSJ3 Goku >= Fat Boo (Suppressed)

You are just twisting the narrative at this point, its outright stated that absorptions weakened Pure Boo

Chapter: 508 (DBZ 314), P4.2-7
Kaioshin: “In m…my era there were five Kaioshins…until they were defeated by the Majin Boo that the wizard Babidi created…I was the youngest and most powerless one, but I somehow survived, with only heavy injuries…But the other four fell to Boo…First, two were killed…the North Kaioshin and the West Kaioshin….And then the South Kaioshin, the burliest and strongest one, was…somehow absorbed by Boo….. “
Elder Kaioshin: …And he became that huge Boo from before? “
Kaioshin: “…Yes…Next to be absorbed was the fat but kind and gentle Dai-Kaioshin….The Majin Boo that Bibidi created was evil itself, a failed creation that even Bibidi himself couldn’t handle, but by absorbing the Dai-Kaioshin, he somehow become controllable…. “
Elder Kaioshin: “…So he was finally completed…Which is to say…that this current…small Majin Boo…is the initial…mo…most troublesome one…”
Kaioshin: “….Yes…the heart which he gained by going so far as to lower his power through absorption[s]…has returned once again to the way it was…he has no self-control whatsoever…he has become evil itself…”

This is very easily contradicted by the fact that when he absorbed Gotenks and Gohan he became vastly stronger.

I think people are misreading what's going on here. Kai is saying his mentality is less sinister and thus is weaker in terms of that. With Kid Buu he no longer has any restraint on his malice and he'll do anything he wants without care, thus that makes him stronger in a sense.

Those are added external power ups but after they are removed his Ki goes down again. Also, I don't know where you got that theory from since he outright says "Lower his power through absorptions" there is no alternative way of interpreting it, this is direct explaination for why his Ki skyrocketed instead of going down after Good Boo was removed despite it going down everytime Piccolo, Goten, Trunks and Gohan were removed.

You know, I wouldn’t be surprised if the DBS manga addresses Great Kaioshin having magic suppression power just to retroactively explain how Boo was weakened after his absorption. So, South Kaioshin somehow having contribution in this process wouldn’t jive well with this reasoning. Not that his absorption mattered anyway.

People don't realize this simple thing. No matter how much South Kaioshin powered up him according to their headcanon, Dai Kaioshin brought his power more than his original form in form of Fat Boo. So the Fat Boo who fought Majin Vegeta was Boo with both Kaioshins in him, yet overall result was downgrade. Which means whatever South Kaioshin provided according to headcanon is inconsequential since his power went down below original due to Dai Kaioshin. That's why, after Vegeta removed Good Boo from inside Super Boo's body, it can't go down since the power he lost from Dai Kaioshin was more than the power he suooosedjy gained by South Kaioshin.