Whenever a siege server has less than 50 people or so, attackers seem to always win. This is because defense needs enough people to crowd the choke points, and with the long respawn for defenders, this is nearly impossible. I know I avoid low pop siege servers for this reason.
If the server admins could just set the defender respawn timer to progressively decrease with lower population (OR increase the attacker respawn timer inversely proportional to population), the problem would be solved, and people would want to play on lower population siege servers.