In viewing animals in low light which is more helpful magnification or exit pupil?
This is a question with diminishing returns on either extreme. Short answer, I prefer exit pupil slightly larger than what I would expect in the light in the given situation. Any larger would be pointless, I reckon, though others may have a better answer.
Twilight factor (TF - square root of mag times aperture) is a good indication of what will be visible, but to me it is counter-intuitive. Say you have a 10x50 binoc which has a TF of 22, and a 7x42, which has a TF of 17. According to TF, the 10x50 will have more ability to discern detail. However, I would take the 7x42, due to field of view and the fact that I'd be able to discern motion more easily. Both should provide plenty of exit pupil. Then some would prefer the 10x50 due to the ability detect smaller movements or finer gradations in contrast (aka critters with natural camo). Different strokes. I guess with long distances, the 10x50 would still be more preferable.
Under most conditions (light beginning to fade at end of day, deep shade, etc) & if the comparison is a reasonable one (e.g., 8 x40 vs 10 x40), I would say magnification.