Is It Bad To Have A Fatty Cat?

Well Recentley, My Fat Cat Got A Bit Fatter....Is It Bad? Or Should I Just Make Him Exerscize Or Give Him Less Food? Its Like A Jelly Belly....I Was Actually Swishing it around and he got mad. Lol. He Doesnt Have Worms, Because I Actually Gave Him Some Pills For That A Few Weeks Ago. Then Again, I Gave him extra food since hes been sleeping in my room recentley and I Lock The door so he cant get out, So I Have A Box,Water, And Food In There..... Should I Be Taking Him To The Vet? o: