Or the game simply says that balanced teams have balanced stats on their gear. Numerical imbalance dynamically produces gear imbalance. Facing a 2:1 numerical disadvantage? You team now has a 2:1 gear advantage. Your shots do twice as much damage. And/or your sensors are twice as sensitive. And/or you have twice as much ammo. And so on. The actual numbers would be based on a perceptual 2:1 gear advantage. Even having a 10% greater damage per hit might be considered twice as good as the defined standard damage.
Because everything is rebalancing continuously as folks arrive and depart, when that guild of 20 guys decides to leave your team, you start to accumulate effectiveness over the next couple minutes. The time delay would be intended to reduce the effectiveness of exploits.
Folks will switch sides in a heartbeat, until some balance point is reached, perhaps only a few percent. However, I can easily imagine teams seeing a 10 or 20 percent advantage for the other team and refusing to switch sides, considering it a greater challenge for their team.
That numerical advantage can be based on player ratings as well. If the sum of player ratings on one team are 10% lower than the other, then they get a 10% gear advantage.
The gear advantage system could also be used for handicapping individuals based on player ratings. If a server is defined to be level 8, then folks rated below 8 would acquire a gear advantage while those above 8 would see a gear disadvantage. The numerical imbalance stuff would be applied on top of that. So a full team of 10s against a single 1 on an 8 server would be... interesting.