Extremely Fat

Listen up, buttercup. This "ur mom" joke is all about saying someone's mom is super, super fat. They're saying she's so big that even Jon Brower Minnoch, who was a real person and the heaviest dude ever, would look skinny next to her. It's like, a major exaggeration to be funny. You probably have a hard time understanding the joke because you are probably "fun sized".