Doesn't it really boil down to pixel pitch and how many pixels you can get on your subject?

The 7DII has a lot of pixels on a smaller sensor (20.2 megapixels on a 22.4 X 15 mm sensor ) so each pixel is very small compared to say the 1DX (18.1 megapixels on 36 X 24mm sensor)

When you take an image like the birds you took, the 7DII will have a lot more pixels on the bird itself than a full frame camera would.

With more pixels on the subject the IQ holds up much better when cropping...thus the "illusion" of more reach.

Is this thinking correct?