Things get compromised more with lower quality scopes,but even with the ideal optics,look at the relationship between power and objective lense.Look at traditional binoculars.7x35's,8x40's,10x50's.Night-marine binocs,7x50's.
When you divide the power into the objective lense size it will give you the exit pupil size.Note the traditional binocs it comes out to 5mm,and the night/marine it is 7 mm.This is matched to the size of the pupil in the human eye.In low light,we have about a 5mm eye pupil.At night,we go up to a max of 7mm.Very bright light,we go down to maybe 2.5 mm.
You will notice a drop in image quality when the optic exit pupil is smaller than the eye pupil.
I have a pretty good spotting scope.Its a Kowa with a 77 mm flourite crystal objective lense.it has a 20-60 eyepiece.Looking at bullet holes at 300 yds,things just do not get better than something near 40 x in very good light.
According to this theory,with a 50 mm objective,and ideal light,about 20x is all that will provide better image quality.More magnification will zoom it up,but its like overzooming a photograph.Image quality does not get better,field of view just gets smaller.