The Gallic side can use transports. The Bretonian side can only use freighters.
Largest transport cargo hold: 5000.
Largest freighter cargo hold: 650.
5000 is about 7.7 times more than 650. Thus, the side that can use transports is 7.7 times more advantageous than the side that can't, i.e. it is as if it has 7.7 times more players.
But I think that he forgot to include the factor that most players will help the side that isn't committing a genocide, which means that the side that is might need a multiplier of 7.7 for the sake of balance.
However, when looked into a bit deeper, it is safe to conclude that there is a lot less Gallic citizens on Leeds than Bretonian ones, as the planet was already populated by billions of Bretonians when the Gauls came, and there is little reason for Gallic citizens to settle on such a hellhole en masse, which means that there should be a lot less evacuated Gallic citizens at the end, too.