The problem:
Zergs flow from campaign to campaign with no penalty. They cap the map from alliances with less than 8x their numbers gating the other alliances then they leave the campaign for the next.
Suggestion:
To keep the non-population locked campaign's population equally distributed there needs to be queue times or some sort of barrier to entry. For example, if the number of any given alliance is 3x greater than the population of the others than a queue time needs to be put in place to slow the growth of the Zergs.
Another solution would be to get rid of a campaign to consolidate populations.
There are too many bad players in this game that just alliance swap to red in order to outnumber their opponents 8:1. Getting zerged down by a massive army of tanks and healers is not fun.