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Exit pupil is the limiting factor. Assuming good glass and coatings a 4mm exit pupil is the standard for the most light to the eye. Anything more is simply lost as the eye cannot receive it.
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I've heard its closer to 8~9m. Relative light transmission matters as well. sending 1/2 as much light through the same exit pupil is way dimmer.
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In bright light, the human pupil has a diameter of about 1.5 millimeters, in dim light the diameter is enlarged to about 8 millimeters.
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http://www.sciencedaily.com/articles/p/pupil.htm
Incidentally you can do a quick & dirty calculation by dividing the effective objective lens diameter by the magnification. based on that alone & ignoring everything else the Nikon would be brightest, but only at the lower 2.5X power.
(42/2.5= 16.8), as opposed to the Zeiss' 3X (42/3.0= 14)
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