Assuming the scopes have the same quality glass, the larger the objective for a given power will typically allow more light to be transmitted to the eye.
Tube size will also play a factor - if both scopes have the same quality glass and the same objective diameter but one has a 1" tube and the other a 30mm tube - the 30mm tube should allow more light through.
Fixed power scopes will also typically be brighter than a variable with the same max power/tube size/glass quality due to having less lenses to go through.
With all that said, sometimes it comes down to individual eyesight being able to actually "see" a difference.
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