There is not an exact amount of light that can pass through the 7mm exitpupil.
The bigger the fronlens and the bigger the magnification the more light will pass through it.
Otherwise if the fact should have been the opposite and a 4x32 or a 6x42 or a 10x72 let through the same amount of light to the eye even a 1x7 or a 2x14 would allow the same amount of light to the eye.
In such case there would be no need of optics for low light hunting, but there is, mostly times you cannot see the animal with your bare eyes but there is no problem to see them with a good scope or binocular.
That said the most important factor is of course the quality of the optical system.
And also that the optical system is optimized for low light situations.
All optical systems cannot be optimized for all situations, and such things as colour correctness even though its nice to have it's not the most important in a low ligth system as you cannot see the colours anyway.
It's quite fun to see how the best of the classical optics that are made for night use are very close to the best systems today when compared at night.
When comparing them daytime the classical systems fails to the modern systems because the have been optimized for military low light conditions 70 years ago, and not for perfect colours or edge sharpness.
So a 1,5-6x42 of good quality is far better than a low quality 8x56.
But of the same quality the 8x56 wins on a low light situasion on 100 yards.