Unlike land based maps which could be 30mb-50mb in size , sins maps are just a set of instructions as to where planets should go so should be easily auto-transferable either in the lobby , a vault system or just before the game loads.
Agreed -- Aerolian sector, the 10 player multi-systemic map, comes in at 46 KBs in size, and is the biggest map available in terms of size.
Edit:
That said, more "official" maps is still a very needed idea.