I have a theory about Boo's strength. Everybody thinks it has to do with who he absorbed. I think that's just a single factor. Instead of it being ki growth alone (I'm assuming ki refers to the source of all forms of typical DB strength in this argument), I also believe Boo manages to draw schtinkloads of ki from his being evil. In contrast, the kaioshins could be doing the opposite and get their ki from goodness. It could also explain why Kaioshin was so shocked by Gohan+co's strength: non-deities would be getting their strength by developping their ki through physical+spiritual training.
Now when blending this evil/goodness with "normal" (non-deity) ki, we're getting into 00ber powerful levels of strength.
1) Kid Boo: said to be most evil form. Therefore he gets the largest amount of evil-ki. We'll label him as a "15" in terms of overall strength.
2) Boo (absorbs South Kaioshin): South Kaioshin is said to be the strongest of the 4, so we'll put him as a 7 just for kicks. After having absorbed him, Boo would gain only most of the South's overall strength since a Kaioshin is very good. Let's say Buff Boo is only "20" instead of 22 (15+7)
3) Fat Boo: It's said that the Dai Kaioshin GREATLY reduced Boo's strength because of his goodness. Here we'll place him at 4 as a result. Big drop, I know, but I'm assuming by Kaioshin's maneurisms that Dai Kaioshin was more good than Boo was evil.
4) Skinny Boo: Skinny Boo was said to be made up of Fat Boo's evil side, which wasn't much at this point. That's why, IMO, Kid Boo wasn't expunged instead. Since Skinny was said to have taken most of Fat's strength, we'll put skinny at between 2 and 3, and the new Fat at what's left out of the original 4.
5) Super Boo: No theory on how Super is stronger than the first version of Fat. This might proove my theory wrong since it should just result in a weaker version of Skinny. Maybe it's because he's absorbed another Boo. We'll clock Super at 5.
6) Super Boo+Gotenks+Piccolo: Here's where normal strength only comes in. We'll say it doesn't assimilate in the same fashion as goodness or evil, so although SSJ3 Gotenks is probably also a 5 (and Piccolo less than 1. Should have used a larger scale), Adding them together would only get 8.
7) Super Boo+Gohan: Now Gohan I'd clock at 7. When he gets absorbed the same thing happens since Gohan's strength only is what gets absorbed. We get what most people argue is the strongest form of Boo, Super Boo+Gohan, who I clock at between 12 and 13.
Buff Boo reappears, making everybody shit bricks because of his "20" strength. People start feeling better when he transforms again into Boo since his ki is reducing. They feel safer now why? Because his ki is coming from evil alone, not from strength (and he's weaker than Buff), so they assume he's someone they can take on. But at 15 instead of 12-13, Goku finds out that he's not strong enough to win like he thought he was in SSJ3, and that he must go full power to win. Surprise! He can't go FP in the living world.9) Boo dies how? By an attack that strikes at the evil in you. Where does Kid Boo's ki come from? His evil.
Now before anybody goes all "OMG TATS @11 L133S~~~!! SH0W MEH IN DA SPANISH SUB W3r3 3xActyl IT SAYS TAT 0r 1ts n0T C@n0n~~~!!!oneoneone" on my ass, please note that this is a THEORY to explain that Kid Boo's and Dai Kaioshin's (and possibly the other Kaioshins') strength comes from a factor not shown in the rest of DB: goodness and evil, and that Boo's assimilations aren't as strong as they appear as a result of this lack of evil in his assimilations.
My thoughts on strength origins:
Physical, Spiritual, good/evil. Up to the end of the Piccolo arc we only see physical, then we see only physical+spiritual up to the Boo arc.
Most questions are about that saga.
