One of the most common antonyms for the word "big-bellied" is "thin." Other antonyms include "slender," "svelte," "lean," and "skinny." These words describe someone who is not carrying excess weight in their midsection. Another possible antonym for "big-bellied" is "flat," which describes someone whose stomach is level with the rest of their body. It is worth noting that while "big-bellied" has negative connotations, these antonyms are not necessarily positive, as they can carry their own judgments and biases. Ultimately, the choice to use any of these words depends on the context and the intended tone of the conversation or piece of writing.