Was South Kai really the strongest?

[Hector]

Why did Burly Buu bother to absorb Dai Kai if the latter was just a small fry? Furthermore, in the anime filler scene, Dai Kai puts up a good fight against Burly Buu. And doesn't it make more sense anyway for the leader of the Kais to be the strongest?

 

[SSJ2]

He can be weaker than South Kaioshin while not being a small fry.

 

[Fantastische Hure]

Zako