I wouldn’t say the World of Ruin counts as post-game. Post-game is usually sonething that is unlocked after the final boss or is at least intended to be faced at a higher level than the final boss. The WoR is just a less linear second half

So basically, extra dungeons, superbosses, extra hunts, added story chapters qualify.

But yeah, if we don’t count remakes, I’d say XII would actually win this, as there are two entire areas that are intended to be beaten in the post-game, and a crap ton of other stuff unlocks too. FFXV is a close contender here as well.