Why is Kog'maw's percent damage maximum instead of current?
He already gets increased range and he doesn't need to hit a target three times for it to actually happen like Vayne has to. Varus's W is similar except that it's not on hit, it has to stack first, but only 3 times, and you have to land an ability which are all skillshots, he is also immobile just like Kog'maw.
Where is the counterplay in a champion who has a Black Cleaver (Q) which doesn't need to stack, Rapid Fire Cannon (W), a better BoRK (W), and a Frozen Mallet (E) all in the same kit and can build above that? If you count BoRK he already gets 7% maximum health (magic damage) AND 8% current (physical).
Like what else are you suppose to build other than 
? Which only stops 20% of the damage, and I can tell you 20% of 7% isn't very much.
