Kog'Maw's W Damage Cap vs Minions & Monsters

I do several hundred in magic damage vs Supers when I have Kog's W on. Since they have -30 MR, I expected extra damage but not 300+. Could the -30 MR really be responsible for all the extra W damage? The LoL Wiki claims that it's capped at 100 vs minions and monsters. This isn't a new thing so what's going on? Do Supers not count as minions? Is this just the most obvious bug in the world or is it not actually capped?

After testing: It does seem to have caps. A maxed W should deal more damage vs Blue (2000 HP) than vs Red (1800 HP) but it does 100 vs both. Even if I buy a Rabadon's, the magic damage remains 100.

It seems that Supers are the only inconsistency but why? Do they not suffer from caps in general or is this Kog specific? For example, would the BoRK cap apply to them?