there are champions who are supposed to stack massive defense and deal that much damage. theyre called juggernauts. but thats besides the point here since mao is a tank.
those damage values are difficult to judge, since theyre flattened out over the course of an entire game.
if mao lanes against mundo and he keeps throwing saplings into bushes, while mundo facetanks those saplings and keeps stacking health, then the mao will deal an assload of damage that stacks up over the course of 20 minutes, but barely even scratches the mundo who keeps outregenerating the poke.
a marksman can deal little damage for 20 minutes while hes playing like a bitch, then suddenly wreck an entire team and end the game. he will still have little damage dealt compared to the maokai when you add everything up over the entire game. but the maokai would have nowhere near the same kill pressure.