There are fewer lenses in a fixed power scope which "theoretically" should mean they should be brighter cause each lense looses or lessens light transmission. That usually doesn't add up to something the eye can actually distinguish though. Something John Barsness stated not long ago and that is that manufacturers put more "R&D" into variable power scopes cause that's where the money is for them.
Talk to JGRaider who recently bought a 6x42 FX3 cause he had a chance to compare it side by side to a Zeiss Conquest and I believe he thought the FX3 was somewhat brighter than the Zeiss plus the fact that his Leupold had the heavy duplex reticle which helps more in low light than minor optical differences. I had a Leupold 1.75-6x32 with the heavy duplex reticle and despite the rather smallish objective lense size, that little scope was pretty good in low light.
