Rotund describes anything that's plump or round, like a teapot or your chubby Aunt Agnes.
Rotund describes someone who is round in shape, or obese. It's not a compliment. That's why it's probably okay to call Santa Claus rotund, but not your neighbor — at least not to his face! It's fine to call round things rotund, however, like a rotund vase full of flowers, which particularly makes sense when you know that the root of rotund is the Latin word rotundus, meaning round, circular, like a wheel.