It would depend on how "much more" the Home Guard would need to do. If a new Member came to the Federation with a significant number of colonies (lets say thirty), then they would need a home guard capable of patroling not just the original star system, but all the star systems where there are colonies, plus the travel routes between those star systems. Earth apparent continue to expand it's number of colonies after becoming a Federation Member. So not only would a Member have a home fleet, there could would be a need to increase it's size steadily over time. There could be other factors involved, other than environmental. it's possible that psychological difference come into play as well. Some Federation species might simply not be able of serving with each other over a protracted period of time. A Human serving on a Vulcan ship for months and years could eventually experience clinic depression through being socially isolated. Although possible not a Federation Member, most Federation species could not serve on a Medusian ship, they couldn't directly interact with the crew.