An APS-C sensor equipped EOS R camera mentioned again [CR1]

[Don Haines]

You are getting confused between field of view and image size.
You put a 400mm lens on micro fourthirds, on APS-C and on FF and take a picture of a bird. The image projected on the sensor will be the same size on every sensor - high school physics will tell you this.The only difference between each format is how much stuff round the bird you will see in the viewfinder.
'Reach' comes from the number of pixels that cover the bird.

Exactly!

Take a 7D2 (crop) camera and a 5DS (FF) camera and put a 400mm lens on each. Go take a picture of a Chickadee that is 50 feet away. If the bird is 3000 pixels wide on the crop camera, it will be 3000 pixels wide on the FF camera because both cameras sample the image at the same density. The 5DS just samples more from the sides of the image.

Take a 90D and a 1DX2 and try the same. The framing will be the same as the above cameras, but the number of pixels on target will be different. With it's lower sampling density, the 1DX2 will have an 1875 pixel wide bird, while the higher sampling density 90D will have a 3815 pixel wide bird.

In general, crop gives you more pixels on target, but FF gives you better quality pixels on target. Your lighting conditions and lens quality will dictate which is better.