Yi vs Kog isn't really a hard counter situation. It's simply a fed gets more fed situation. A kog with more items will out damage a Yi. But don't worry, all that free gold you gave him only goes towards Backdoors

Problem: Yi cannot be slowed by Kog'Maw
Solution: Kill Yi

Kog'Maw has much better range, and pretty massive damage on his own.

If you, as a Kog'Maw, are having difficult killing a Yi, you purchase a Thornmail. Now he deals no damage to you, 30% of that to himself, and you still deal impressive damage of your own.