Why did Usopp get fat?

Usopp is a character on the Japanese manga series, One Piece, who gained a significant amount of weight whilst on Boin Archipelago Island, in order to build muscle mass.

Usopp is an iconic character on the popular Japanese manga series, One Piece, which is written and illustrated by Eiichiro Oda. Usopp is the sniper of the Straw Hat Pirates and the former captain of the Usopp Pirates. Fans of the fictional series often make reference to the character’s significant weight gain during one of the earlier chapters of the series, which is often seen in many of the One Piece fan art and illustrations.

His weight gain occurred during his two-year training period on Boin Archipelago Island, on which a huge carnivorous plant is found. Upon tasting the plant, Usopp could not control himself and continued to overindulge. After receiving a message from Luffy, he decided to adapt his training regime to accommodate his love for food. He began running around to lose the weight, and then eating again thereafter. After a while, he managed to build a significant amount of muscle, which proved to be to his advantage in his future combats.