User blog comment:Makbloo/Grolk Kar, The Great Wyrm, Kog'Maw's Daddy/@comment-83.83.172.54-20121208100804/@comment-5127158-20121208111245

Garen's W increases his Armor and MR by 20%.

For Grolk's innate passive to be useful you have to farm a lot, since every last-hit gives only 0.2 Armor and MR (5 last-hits for 1 Armor and Mr). Additionally the passive caps at 100 bonus Armor and MR, which would require 500 minions to be fully stacked. Well I don't think that even Froggen could fully stack this passive.

An anerage player would have around 200 minions (solo-top) by late game. This would give 40 bonus Armor and MR. Contrarilly at late game Garen's passive on his W would give also him 40 bonus Armor if he only has 200 Armor (which he usually has) and 20-40 MR depending on his build. Taking into consuderation that Grolk's ability to increase his Armor and MR is his passive, while Garren ability is passive of one of his skills, I don't think that this innate is in any way overpowered. :)