... and amount of eye relief, and amount of reticle adjustment in MOA, and variable power, and parallax at different distances, etc.
Generally speaking, for two rifle scopes with the same quality of glass, and with the same magnification, the difference between a larger objective lens and the smaller objective lens is not brightness of image but field of view.
And that still increases brightness as more light is hitting the objective lens. Larger FOV is a function of the arc of the lens as well as diameter of the objective lens.