Michael Klump is one of the many alter egos that Michael Scott created. He uses it to make fun of fat people. Michael Klump's catch phrase is "I say, I say, I'll sit on you!"
Michael Klump is first introduced to the show when Michael comes into a meeting telling everyone that if you are obese, you will die. Holly Flax is shocked, but the rest of the office recognize Michael Klump as a character Michael uses to make fun of fat people. In the episode Weight Loss, Michael repurposes the character as a celebration of fat people.
To assume the character of Michael Klump, Michael puts on a partially-inflated sumo suit (which he obtained in the episode Beach Games), then wears plus-sized clothing over it. The name "Klump" most likely comes from the morbidly obese character Sherman Klump in the 1996 version of the movie The Nutty Professor.