Dumb ? about low light.

I kinda understand(alright, not really, but I have accepted it as fact) how a lower magnification in a given objective size will be brighter in low light conditions than a higher magnification in the same objective size. 

My question is this, Why can I see deer better in low light with my scope set on 9x vs 3x?   I realize it looks bigger, but it also appears brighter to my eyes.  Any one else notice this?  Can anyone explain it?

From what I read everywhere the deer should look like daylight at 3x and be much darker at 9x.  It appears backwards to me.  The scopes in question are a nikon monarch 3-9x40 and a burris fullfield II 3-9x40. 

what do you need to know about the 2 scopes in question??

Nothing, thats just the two scopes I have that I have noticed this with. 

If the exit pupil coming out of the scope is larger than the pupil in your eye then you are not going to see any brighter of an image.  at 3x the exit pupil will be 13.3mm where you eye is probably only going to dialate to around 5mm maybe 6mm at night.  so all that extra light coming through is not going to make the image brighter.  At 9x the exit pupil will be 4.4mm.  In your case your eyes might not be dialated any higher than that so the image brightness will be the same but you will be closer to the image so it will be easier to see. 

Ahhhh, so the scope can be too bright to make a real difference?

What is being said is that if the exit pupil of the scope is greater than the diameter of your eye pupil, the extra light is wasted. Unless you are in your twenties or are in exceptional health, the maximum your eye pupil will dilate in darkness is 7mm. During low light hunting conditions your eye might only dilate to 5-6 max. So if you set your scope to it's highest setting that does not exceed your eye pupil diameter, that is where it will show you the brightest image.

Low power is good for fast scope picture/target acquisition.  1) The Field of View will be larger making it easier to find the target and 2) the larger exit pupil will allow you to pick up your cheek weld faster.

They have hit it pretty well. I will add that the extra resolution added by the magnification will help you to see the target better. This may make you think the image is getting brighter, when you really are only making details out better.

In optics, ultimate resolution (maximum distance between two points that can be resolved on a surface) is proportional to the distance from the observed object, and the ultimate aperture through which the light passes.  In the case of variable scopes, the human pupil is USUALLY the limiting factor, but under some conditions the exit pupil will act to reduce resolution.