You want conjecture?
Didn't try to lift the Z-Sword? Pffft... he was obsessed with it.
The sword itself is shown to be magically enchanted and not actually heavy (due to Goku's handling of a much larger, and supposedly denser Katchin cube).
So, obviously, the sword was enchanted to be heavy... but the enchantment was strongest against Shinjin (which would make sense considering where the sword was - sacred world of the Supereme Kai... no mortals allowed). The magic was less effective against filthy ningen like Gohan and Goku.
South Kaioshin wanted to be strong. Whatever the reason... he needed strength.
The Z-Sword was the ultimate symbol of strength. To lift it was not only to be strong, it would also grant even greater strength.
He tried. He tried every single day. His years of obsession with the sword had led to a focus on raw strength. He slowly, passively, unintentionally, acquired a kind of "SSJ Grade 3" style permanent transformation (which is why he was so "burly").
South Kaioshin was way beyond SSJ3 Goku.Stronger than even Ultimate Gohan. Although he possessed incredible power... his focus on raw strength left him lacking in ki control/finesse.
When Kid Buu appeared, and killed the other two Kaioshin, South Kaioshin stepped up and beat the living crap out of him... but without a decent ki attack, was unable to finish him. A desperate Buu, unable to fight back, ends up absorbing the much more powerful SK.
His absorption was not everything it should have been though... sure, Buu's strength shot through the roof... he was now a powerhouse beyond Ultimate Gohan levels... but the unbalance of the strength... and it's sheer vastness was too much for Buu to handle... which is why he quickly turned his attention to absorbing the much weaker... but far more magically/ki adept Dai Kaioshin... taming the great, unwieldy power he now had.
Later, when Dai Kaioshin is removed, and "Buff Buu" reappears... Buu's body once again could not handle the incredible strength of South Kaioshin... and his body expelled the excess power (burning it off in the form of steam).
...I'm taking this as head-canon and there's nothing you can do to stop me...
...though I may write it a little better...
