The term "most unexpansive" implies that something or someone is not prone to expansion or growth. Therefore, the antonyms of this phrase would be words that describe growth or expansion. Some antonyms of "most unexpansive" could include: expansive, vast, extensive, capacious, broad or spacious. These words suggest that something is capable of expanding and growing, and can contain a lot more than what is currently present. While "most unexpansive" may connote constraints, these antonyms project a sense of openness and possibility, indicating that there is room for expansion and development.