Rotundity is the noun for that round, fat shape of a body — round fatness, said in a humorous, slightly old-fashioned way. If someone writes about a man’s rotundity, they mean his plump, circular figure, often with the same little smile as rotund. It is not a word you need at the shop or the clinic; it belongs to stories and playful description. You could, in careful English, use it for roundness in general, but learners will almost always meet it as this jokey body word. The everyday way to say the same idea is just roundness, or a round, heavy shape.