When one side gets a major advantage in number then eny restricts what planes they can take. In the hangar you see that each plane has eny. The "better" the plane the lower the eny (like the spit 16 and la-7), and the "worse" the plane the higher the eny (like the P-40 and 190s).
So when one side has 110 players, and the side with the least players on has 70 and mini-eny is 10.0 for the side with the most players, then that side can't take planes with 10 or lower eny.