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Yes, a 120mm aperture collects more photons than a 100mm aperture, but we do not bring all those photons into a single point on a sensor. If we did, our images would be just one bright point at the centre of a sensor, and larger aperture would make that point, or a blob, brighter. Instead, some/most of the light collected by aperture is being distributed over the entire sensor, and it is the f-ratio, not the aperture, that will determine the brightness of extended objects being projected on the sensor. Stars, on the other hand, benefit from large aperture. And so does resolution of images of the extended objects.
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