Whenever it gets cold, people tend to eat, and its the same with Chopper. Perhaps, by fattening himself a bit, he would have less chance of getting a cold, what would happen if the physician of the crew gets sick? Besides, eating is such a happy business.
Pumpkins become ripe in this season and Chopper is having a wonderful time slurping up some fresh and creamy pumpkin juice. He has certainly plucked a big one, almost as big as himself and is filling himself up nicely. With so much vitamin C and fibers, Chopper is making a healthy choice.