There is only one reason I would ever use 000 buckshot for big game.
Using a pump action 12 bore, in really, really heavily forested cover, this would be the first and only buckshot in the chamber with safety on, or top of the magazine.
The reason, if one is surprized at close range, you have a better chance of hitting big game with one pellet or two or more hopefully, if it is legal to use in this jurisdiction.
This would be the only single buckshot round I would load, the rest of the magazine, the best slugs I can afford.
And if I stepped out into more open terrain, this one 000, would be swiftly replaced with another slug.
My two cents.....