Bulbous describes a shape that is fat and round, like a plant bulb, and the word is rarely kind. People use it most about a nose, sometimes a belly, a vase, or a cloud — anything that swells out in a heavy round way. It is a slightly bookish, descriptive word, not everyday chat; in conversation someone might just say round or big. The “unattractive” feeling is part of the meaning: you would not usually call a baby’s cheeks bulbous if you meant to be sweet.