I'm of the impression that our eyes need brightness AND volume of light, with volume being a bit more critical. Currently, light transmission (brightness) is maxed at about 95%. Quality of glass is very important here as well. However, if you are sending this light through a pinhole, you might not be able to use it (not enough volume).
An example, stick your head in a box and shine a light through a pinhole. You probably won't be able to read anything due to low volume of light; now, cut a 4" hole in the side of the box and you can read a book.
So, a big part of the light transmission through a scope is the size of the internal lens in the erector tube. Small lens = low volume of light hitting our eye.
Check a scopes amount of windage/elevation adjustments. The lower amount of adjustments might mean a larger lens in the erector tube, therefore, more volume of light getting to our eye (all other things, such as lens coatings, being equal).
