Why does Galio have the highest base HP of 2466? (562 + 112/level)
His base HP is 91 more than the 2nd highest base health champion, Alistar, with 2375 base HP.
Paired with the damage reduction from his W and the free passive shield, this makes him rather tanky and able to get away with full AP builds.
Normally, champions with free passive shields or damage reduction in their kits have this factored into their power budget, like Warwick with 1995 base HP and Nautilus with 2038 base HP.
If you want to bring Galio in line, this would be the best place to do it.