You are so fat that when you go out to check your letterbox, it measures 8 on the Richter scale.
Fat and the Richter Scale
Explanation
Explain Bear
Listen up, butterball. This joke is saying you're so obese that when you waddle to your mailbox, it's like an earthquake. The Richter scale measures the magnitude of earthquakes, and an 8 is a BIG one. Basically, you're a walking natural disaster.