The exit pupil size is dictated by the objective size and the magnification; that's just optics 101. The amount of light entering the scope is dictated by the size of the objective; the bigger the objective the greater the amount of light coming in.
Now, that said, while the size of the exit pupil is dictated by the objective and the magnification, it doesn't mean that the exit pupil will have a lot of light or even quality light.
We talk about lens coatings has if they were magical. Well in a way, they are but they are also well understood. Uncoated lenses have about 5% of the light crossing each air/glass boundary reflected. Reflected means lost. So in a riflescope with 5 lenses, the front side of the lens peels off 5%, leaving 95%. Then the rear side of the lens peels of 5% of 95%, and then the front side of the second lens peels off 5% of 90.25% and so on. After passing through the fifth lens, the amount of light is not at 63.02% of when it came in. That's degradation.
Lens camera companies such as Nikon developed coating techniques that reduced dramatically the amount of light reflected by the air/glass boundary, to provide better color fidelity. Since visible light is made up of multiple wavelengths (what we call colors,) there was a need to have multiple coatings to deal with these colors, which were being reflected in different percentages. This is called multi-coating. BTW, these coatings are extremely thin and are applied in some interesting ways. Bing it and read about it.
Other coatings were developed for the objective lens to repel water and other stuff, to harden the glass, etc. Some companies developed various techniques to apply the coating and give them catchy names. I read about that in photography magazines, websites and so on.
Nowadays, the coatings are fairly similar across the board and the light transmission differences between high end scopes are such that it's impossible for human eye to detect. A few years back I read an interesting article in a German magazine that compared a bunch of high end scopes like S&B, Swarovski, Leupold, Docter, Zeiss, Nightforce and Burris, and it included a low end Nikon Monarch scope, just for fun, I think.
These tests were done at a optical laboratory. They measured a bunch of features and qualities but the one that astounded them (and me,) was the light transmission. They measured it in day and night conditions. The highest light transmission figures were the ones for the Nikon Monarch, a $400 scope. It had a day % of 93.3% and a night % of 91.4%, beating out the second place S&B which had 93.1% and 91.0%. The article described their amazement at these figures. Of course, the S&B had better knobs and other features, but this comparison was a real eye opener, pardon the pun.
(Before you ask, this was several years ago and the article is no longer found on the web. I have a printed copy and the translation I did from German to English, somewhere.) But the thing of it is, the vaunted "German glass" was pissed on and chewed up by the Japanese glass, and at a fraction of the price. Nowadays the scope manufacturers all claim light transmission figures north of 90 to 94%. I've seen 95% recently. The point is, they are all pretty much the same and human eye can't tell the difference. So at this point, it's a function of how much light comes into the scope since almost all of it will make it through.
If your riflescope has colors that pop with good contrast, you have great coating and by the same token, a high rate of light transmittance. The next frontier was color fringing, or chromatic aberration and that's what ED glass corrects.
Magnification range, ED glass, reliable adjustment knobs, zoom ratio, adjustment range, those are the things you want to look for in scopes. Get what suits your needs.
