Other forms: tubbier; tubbiest
To be tubby is to be round and plump, and maybe a bit on the short side. Many cartoon characters, including Porky Pig, Garfield, and Homer Simpson, are a little tubby.
The informal noun tubby is usually used to describe a person, and as a result it's almost always offensive — it's just generally bad form to comment on the shape of someone's body! You might, however, describe your plump little kitten as tubby or mold a tubby figure out of clay. Tubby, thought to come from "shaped like a tub," has been in use since the 1890s.