It has nothing to do with the size of the sensor, it's all about pixel density. If you take a FF sensor and an APS - C sensor with the same pixel density and same quality of pixel you can crop an identical quality/resolution 1.6 sized image out of the center of the FF file. No reach advantage what so ever. I believe that was the case back when the 20D was the current 1.6 crop and the 1Ds2 was the current FF sensor, they both had identical pixel size / density.
Of course a FF sensor with the pixel density of the current 7D/60 would be around 47 mp so there will probably continue to be a true reach advantage with the 1.6X croppers.
Blimey, this really DOES complicate things. Your arguments are true, but not really relevant to the core point that the OP makes as they concern resolution, not image size or "reach".
No. The "reach" advantage is a virtual one. A crop viewfinder takes the central 62% or so of the image available from the lens and magnifies it so it is similar to the image size in an FF viewfinder (yeh, I know FF bodies have bigger screens but hey) resulting in an extended telephoto effect. Similarly, images recorded by the sensor have to be enlarged by a factor of 1.6x vs FF to result in the same print size so similar tele effect. Equivalance therefore matters, especially to those of us who use both 35mm/FF and crop bodies and it's fair enough for manufacturers to state this.
The pixel density comparison opens a whole other can of worms and is far more complex than the maths would suggest.
