What makes fizz do so much damage through MR?

I'm honestly curious here. He can literally half health you without his ulti through a spirit visage. I don't really look at numbers, so I can't tell you how much MR I had at that time, but I do know he burns me through a SV with only sorc boots and no other MPEN item.

I'd be fine if he had a void staff or liandries doing it... but only 15 MPEN on sorc boots?

Honestly just curious. Don't have major problems with him outside of that.