Lense quality is far more important than objective size. Some better high end scopes transmit 95%+ of the light that comes through to the eye. Most decent mid-level scopes are in the 90-95% range. I'd say most Leupolds are in the mid-level. Some budget scopes are in the mid 80's.
The size of the objective, along with the magnification determine the diameter of the beam of light that comes through, not the brightness. Assuming equal quality glass a 50mm objective set on 10X, a 40mm objective set on 8X and a 20mm objective set on 4X all let exactly the same diameter beam of light through. Which is brighter will be determined by the lense quality
Long story short, if comparing equal quality scopes in 40 vs 50mm you'll never notice any difference below about 7X or above 9X. The 50mm lense offers a slight advantage only at around 8X or 9X. If you need a scope with more than 10X magnification then bigger objectives start making sense.
