Except in extremely rare conditions a 50mm glass offers no low light advantages. Bigger glass lets in more light, in scopes with equal quality glass, but the human eye cannot use all of it.
Most people's eye will only dialate to 5mm. A 40mm scope set on 8X lets in 5mm of light. A 50mm scope set on 10X lets in 5mm of light. A 50mm scope is only an advantage campared to a 40mm scope when set on 9X. Anything less and they both let in more than the human eye can see, anything more and both let is less than ideal light.
And this is assuming both 40mm and 50mm scopes are using glass of equal quality. It costs a lot more to make 50mm lenses of equal quality to 40mm glass. Most companies cut corners on their 50mm scopes.
The more important factor is the light transmission rating. Most budget scopes transmit around 80% of the light coming into the scope to your eye. Better scopes are around 85%-90%. The high end scopes are 90%-95%. A 40mm scope with around 90% light tramsmission will be far brighter in low light than a 50mm scope with an 80% rating.