I use 00 buck and don't plan to change. (And no, I will NOT volunteer to stand in front of even a skeet gun shooting #9s!)
I say this because a couple of African PH's I hunted with became disenchanted with smaller sizes of buckshot (including the famous English SSG) when following up wounded cats in dense brush.
Forget about lions - they're WAY bigger than people - and think about leopards. Their skin isn't very thick and the animals themselves aren't very large; adult tom leopards will average around 160 or so, and rarely get above 200. People sized, right?
Though leopards are faster and stronger than a human of the same size (not to mention having claws and teeth) if smaller buckshot can't be depended on to stop them, then what about the 6'4" steroid-enhanced biker dude with a fistful of PCP up his nose that's about to start carving you? Especially if he's wearing a leather jacket?
If I knew that I'd be engaging a skinny T-shirt clad bad guy at off-the-muzzle range, even birdshot would do the trick . . . but since a real scenario may be very different, I'll stick to something that will work under any reasonable circumstance.