even if you use haki i don't think you truly hit logias with your full power. no reason why his earthquake power would of hit him 100%. he smacked him down but it probably wasn't enough to truly cause massive damage. plus they are admirals so they aren't going to go down from one hit and i don't expect emperors to go down from 1 or 2 hits. at that point akainu was far stronger than whitebeard and maybe the strongest fighter in the world.
Don't forget that Akainu is ALSO a haki user. He can defend himself with haki to counteract Whitebeard's attacking him with haki. Obviously Whitebeard was still stronger, so he did real damage, but the damage is nowhere near the actual full power of the attack.