My real problem is that giants (12 HD+ Giant-type beings) are the same size category as ogres (4 HD Giant-type beings) or trolls (6 HD Giant-type beings). This seems a bit wonky to me, since were one to think of hill giants as, say, advanced ogres, they certainly would NOT be Large-sized.
Here's a question. What would happen if you increased the hill giant's size to Huge, gave it +4 Str (essentially making the stats identical to those of an "advanced ogre," were there such a thing) and then scaled the other giants up from there?
Well, to answer my own question, you increase the thing's attack rolls by +1 and give it 3d8+12 damage (+6 damage overall) with a Huge greatclub. Hmm. Probably worth a +1 CR bump.
Or we could say that +14 Str (the hill giant's Str modifier) is okay for a Huge creature and leave the hill giant's ability scores as they are while still raising its size to Huge. That bumps the damage by +4.5 at the cost of a -1 to attack rolls, which is probably not a high enough net benefit to warrant a CR increase, especially with the -1 AC as well.