I have found that the F4U Corsair is the only aircraft without this problem, and that is due to its extra large glass panel the crosshair is projected on. So it seems the problem is that the crosshairs are too big for the glass they are projected against, but there is no way to make them smaller unless you go with the approach of the guy before me, use a red dot sight. Other than that, I have found that moving up very close to the crosshair while flying, almost putting your face against it in the VR world, makes it easier than sitting back and using it to shoot.
If anyone has any other suggestions please share, this can be a real pain in most planes with smaller glass gunsights. I've missed out on quite a few kills because of this.