SpongeBob SquarePants - "I just realized... how incredibly strong Patrick is if flats couldn't even dent spongebob... but Patrick gives him a black eye in one swing... what the f---"

Patrick lives under a rock that he literally lifts up every time he wants to go in or out.
Flats probably just used his size to his advantage.

It should be pointed out that Patrick punched Spongebob in the eye while Flats went for Spongebob's body.
It's posible that Spongebob's eyes don't have the same ability to take punches.