Heftiness is a word that describes a physical attribute related to weight and size. Its antonyms, on the other hand, are words that describe the opposite qualities, such as lightness, slenderness, and thinness. These antonyms are often used to describe people or objects that are not heavy or bulky, but rather delicate or graceful. Another common antonym for heftiness is agility, which refers to the ability to move quickly and easily. By using these antonyms, we can provide a more nuanced and descriptive language to talk about the physical characteristics of people or things, and avoid reducing them to a single attribute such as weight.