The antonyms for the word "over-fills" can be "under-fills," "insufficient," and "under-stuffed." These words describe the opposite of filling something to the point of excess or overflowing. Under-filling refers to not adding enough material to an object, container, or space, leaving it either empty or insufficiently filled. Insufficient means not meeting the required amount, quantity, or standard. Under-stuffed refers to stuffing an object or space with less material than it can take, resulting in a less than fulfilling experience. These antonyms emphasize the importance of balance, moderation, and adequacy in filling objects or spaces.