Not necessarily B41 bomb ca.1960 25Mt compared to modern B83 bomb 1.2Mt. The B83 isnt even more efficient than a presumably ancient weapon designed in the 1950's.
Given that the UFP is run by pacifistic space hippies, it would seem to be entirely out of character for them to develop more powerful weapons, considering that the US and Russia havent, and both are much more militaristic than the UFP