I still believe that the MACO were not a large fighting force. We know that the Earth nations still had militaries, so the MACOs were an elite fighting unit granted to the United Earth government. A small force it always had access too.
You mean one of Earth's nations loaned
a squad of their special forces to Starfleet?
I kind of like that, and it would explain Archer's first reaction to be told about the MACOs.