Actually, I prefer low player densities, as I perceive (rightly or wrongly) that I'm more likely to find a 1-1 in those situations.
However, if you insist on reducing map playing area based on arena population, make this applicable just to AC, allowing GVs to play anywhere. Then the AC guys will get the compressed play area (for what that's worth), and the GVs will get the equivalent of additional tank town space (i.e. the portion of the map which has been made off-limits).
MH