By using an APS-C camera with higher pixel density, you will get "more pixels on the subject" than with a FF camera with lower pixel density.

Sometimes that can be a good thing.

There are many wildlife photographers who use APS-C cameras with telephoto lenses for this reason. It allows one to get more "reach" with shorter (and less expensive) lenses without taking the light loss penalty of a 1.4X or 2X extender.

Take, for example, the Canon EOS 7D Mark II. It has 20.2 MP in a 22.4 x 15.0 mm APS-C sensor. A 4.09 µm pixel pitch equates to 51.7 MP on a 36 x 24 mm full frame sensor. Interestingly enough, that's just slightly more than the Canon EOS 5Ds/5Ds R with 50.6 MP.

So if one were comparing the 20.2 MP 7D Mark II to the 50.6 MP 5Ds, there would be very little difference in terms of how many pixels the same subject at the same distance seen through the same lens would occupy on either sensor.

What would be different:

• Cost: The 20.2 MP 7D Mark II sells for well less than half of the cost of a 5Ds
• Speed: The 7D Mark II can shoot 10 frames per second for up to 31 raw images or as many jpegs as the memory card can hold. The 5Ds/5Ds R can only shoot at 5 fps for 14 raw or 510 jpegs before bogging down. This is mainly because of the increased processing load of 50.6 MP images versus 20.2 MP images.

On the other hand, if one is comparing the 20.2 MP 7D Mark II to something like the 22.3 MP 5D Mark III, then there would be a noticeable difference when viewing images at 100%. There would be a 53% (1.53X) advantage linearly, and a 2.33X advantage in terms of area.

In the end it comes down to the pixel pitch of each camera, regardless of the total size of the sensor.

As for lens performance of the same lens on an APS-C camera versus on a FF camera cropped to the same dimensions of an APS-C camera: There would be no difference whatsoever. Both images would be using the same exact part of the image circle cast by the same lens. Assuming the resolution of the lens is not the limiting factor, then the sensor with more pixel density (smaller pixel pitch) should give better performance.

The original EF 100-400L IS is a pretty good lens but might be the limiting factor with very dense sensors such as the 7D Mark II or the 5Ds. The EF 100-400L II IS is enough sharper that it should not be the limiting factor with any of Canon's current sensors in EOS DSLRs or EOS R mirrorless cameras.