Here is the problem with big lenses. The size of the lense "DOES NOT DETERMINE" the amount of light going through the scope. That is determined by the QUALITY of the glass. When light comes out of the rear of the scope it is in a circular beam. Think of a flashlight shining on the floor. A larger front objective COULD allow for a larger beam of light, but that beam of light could just as easily be brighter from a better quality, but smaller lense.
But you also have to consider the scopes maginifcation, and how much light the human eye can process. A 50mm lense set on 10X will have a 5mm diameter beam of light coming out the rear of the scope. Most human eyes cannot handle more than 5mm. If you are still in your 20's, with above average eyesight you might be able to handle 6mm. Any more is simply wasted. Incidently a 40mm scope set on 8X will have exactly the same 5mm of light.
Assuming you are using glass of equal quality, and 50mm lense is a lot more expensive than a 40mm lense, a 50mm lense will offer a slight advantge over a 40mm scope only when set on 9X. And only for 5 minutes or so each day. At any magnification lower than 9X you are getting more light, but most eyes cannot process it. Any magnification greater than 9X and you have no advantage over a 40mm lense.
It is simply not worth the extra expense, added weight, and higher mounting to buy a 50mm scope.