Antonyms for the adjective "more well-built" include words like frail, delicate, fragile, weak, and feeble. These words suggest a lack of physical strength and robustness. They are often used to describe people who are sickly, elderly, or injured. Other antonyms include adjectives like lean, slim, slender, skinny, and emaciated, which imply a thinness or lack of muscle mass. These antonyms can be used to describe people who are undernourished, malnourished, or suffering from a medical condition that affects their weight or body composition. Ultimately, the opposite of "more well-built" depends on the context and the specific qualities that are being compared.