I am going to stick my head over the parapit and put in my reckoning on this.
Flame me if you think I am wrong. You might be right. I am not an expert.
As I understand it,two factors define the capability of a scope at twilight.
Twilight Factor (square root of (mag x objective diameter ))
Relative Geometric Brightness (square(exit pupil))
Twilight Factor indicates resolving power and RGB indicates ability to gather light.
Multiplying these gives a combined measure which I will call Twilight Capability.
If you calculate the Twilight Capability of a 50mm scope and a 40mm scope at various magnifications, and allow for the stopping down caused by the human eye pupil, some interesting numbers appear.
We can easily compare the numbers by dividing the Twilight Capability of the 50mm scope by the Twilight Capability of the 40mm scope. Lets call this the Relative Twilight Capability.
We can see that at low magnification, there is very little difference between the scopes. The Relative Twilight Capability is
1.118 giving a slight edge to the 50mm.
This is because the eye pupil (say 6mm) becomes is limiting factor at low magnification when the scope exit pupils are large.
At high magnification, the scope exit pupils are small and the eye pupil is not a limiting factor in both scope.
When this happens, the ability of the 50mm scope to gather more light gives it a distinct advantage over the 40mm scope.
At 10x mag. and greater, the Relative Twilight Capability is 1.747, indicating that at higher magnification, the 50mm scope is 1.747 times more useful than the 40mm scope.
It is the median area between say 5x and 10x strange things happen as the eye becomes a limiting factor. Different people will have diffent experiences with different size scopes depending on their own individual twilight eye pupil size. Also eyes differ greatly. Some might benifit more from resolving power than brightness and visa versa.
I have posted the numbers at the link below if any body wants to see them.
I will email an excel spread sheet to anybody who wants it,detailing full calculations.