So the fact it is a crop sensor is not the reason you are getting the better detail (1.4x sensor size does nto mean a 100m lens becomes a 140mm lens in terms of msgnification at the sensor). It is down to the sensitivity of the sensor which in turn some comes from the pixel pitch/density.
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okay...if you say so.
I tire quickly of semantic distinctions like this. The apparent magnification of the lens is an intrinsic property of the APS-C sensors when compared to FF sensors. You are correct that the image projection that the lens produces remains the same size and that there are limitations imposed based upon the lens quality. But, prints made from an APS-C sensor have higher apparent magnification and more actual detail than full-frame sensors with the same lens and of the same generation.
So, if magnification of your subject is a major concern (getting more apparent length from your long lenses) and you can accept slightly reduced dynamic range and slightly increased noise at higher ISOs, the cropped sensor makes great sense at a much lower cost. Depending on your needs, either APS-C or FF could be a better tool.
So, to summarize my understanding of the tradeoffs between FF and APS-C...
Benefits to FFMore wide angle lens options.
Better dynamic range.
Better high ISO performance.
Diffraction effects are less pronounced.
Liabilities to FFVignetting will be more pronounced when it is present.
Lens distortion is more noticeable, particularly near the edges of the frame.
Telephoto lenses don't benefit from cropped-sensor 'magnification.'
Substantially more expensive (including body and longer lenses)
Benefits to APS-CIncreases the effective focal length of lenses
Decreased vignetting.
Substantially less expensive (including body and lenses)
Lens distortion is less noticeable, typically because it occurs near the edges of the frame that are 'cropped' from view.
Liabilities to APS-CIncreased noise at higher ISOs.
Decreased dynamic range.
Diffraction effects are noticeable a few stops sooner.
True wide angle lenses are hard to find.