The phrase "jammed full" typically describes something that is packed tightly and completely filled. Therefore, its antonyms would be words that describe space, emptiness, or a lack of fullness. Some potential antonyms for "jammed full" might include words like spacious, empty, open, airy, sparse, roomy, hollow or vacant. These words could be used to describe anything from a room or building that is not crowded, to a container that still has plenty of room left inside. Overall, when seeking antonyms for "jammed full", it is helpful to focus on words that describe a lack of density or a sense of emptiness.