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It comes down to exit pupil. A 250mm aperture will give a 1mm exit pupil at 250x whereas a 200mm aperture would give 0.8mm exit pupil at 250x magnification. The object will be brighter at the 1mm exit pupil.
Focal length has nothing to do with it since all it means is that to get 250x with 3000mm focal length you need a 12mm eyepiece whereas 9.6mm is needed to get the same at 2400mm focal length. Of course if seeing permits you can get the same 0.8mm exit pupil at 312x in the larger aperture using a 9.6mm eyepiece.
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