Not neccesarily. The human eye can only use so much light. The light transmission differences between a 40mm and a 50mm will be very small if the quality of the glass is high and the size of the exit pupil is the same. As long as you have an exit pupil of atleast 5mm you should be good to go.

Edit- I should also add the advantages of using a 40mm scope. You can mount the scope closer to the gun, therefore sighting the scope in is easier, the gun is less top-heavy due to the lower mount and lighter scope and the firearm is generally just lighter. There is nothing wrong with using a 50mm scope, but the weighing the pro's and con's makes me want to stick with 40mm.