Is an optics thing, I think - exit pupil is going to be magnification divided into objective lens diameter, so for 6X setting, is one 6th (?) the diameter of the objective lens - so with 42 mm divide by 6 = 7 mm exit pupil. With 36 mm objective, divide by 6 and get about 6 mm exit pupil. So the 42 mm is "brighter", if your eye can use that. I found as I got older that my eye's pupil gets smaller - I doubt that I get to much more than 4 mm any more - so that means the 6x36 puts out more light than my eye can use. Yes, the 42 mm will pass through even more light, but my eye can not use that, either - I think. Younger folk (teenagers) might have eye pupils that open to 8 mm or more - so they would most definitely be able to see the "brighter" image.
Perhaps is why on a 4-12x40 scope - so 12 power is 3.33 exit pupil - most will see that as "dim". 8 power is 5.0 mm exit pupil - many will see that as a brighter image than when set at 12X. 4 power is 8.0 exit pupil - because of our eye's pupil dilation, many may not see much "brightness" difference between 4x and 8x in that scope.
I am not sure about "field of view" - I thought that was controlled by the eye piece lens diameter, but I might have that wrong.