If you look at the aircraft selection screen, you'll see each plane has an ENY value. The P51D is 5, the P47D11 is 35 for example. In general, better plane, lower ENY number.
If your side has enough of a numbers advantage over the lowest population side, planes with lower ENY numbers can't be used. It's based off of percentage of total arena population. It shows up on country status when your side has 10% more of the arena population than the lowest side, but it will be a smaller number than 5. At around 12% or so, it starts affecting what fighters you can fly because the ENY limiter hits 5.
It is capped at 20 if memory serves, so no matter what the system will not limit planes above ENY 20.
Wiley.