Thick as a brick, fat as a hippo, plump as a pumpkin, bulky as a bear - these are all antonyms for the phrase "thin as a rail." Using antonyms can help to illustrate a point or highlight a contrast. For example, if you're describing a person who is painfully skinny, you might use the antonym "chubby as a cherub" to emphasize the difference. Similarly, if you're talking about an object that is impossibly thin, you might use "thick as a log" to create a vivid contrast. By using antonyms, you can create a more compelling and memorable description.