The phrase "more jam packed" refers to something that is very full or crowded. However, there are several antonyms for this phrase that can convey the opposite meaning. Some of these antonyms include "spacious," "uncluttered," "unfilled," "empty," and "bare." Spacious refers to something that is large and has plenty of room to move around. Uncluttered is used to describe an area that is free from an excessive amount of objects or things. Unfilled refers to a space that has not been filled up completely. Empty means that there is nothing inside an area or container. Bare refers to an area that is completely devoid of any objects or things.