Your balls are so big, when people see you at the market, they think it's watermelon.
Balls mistaken for watermelons
Explanation
Explain Bear
Listen up, dummy! This "joke" is saying the dude's balls are so gigantic that they're mistaken for watermelons at the market. It's supposed to be funny because, well, nobody's balls are actually that big! If your balls are that big, then go see a doctor!