[Brian Pfleuger]

You can make it a lot easier to understand by removing the complications inside a rifle scope and just considering a window in your house.

What determines the Field of View out your window?

Only two things, the size of the window (objective diameter) and how far you stand from the window (eye relief).

It's complicated somewhat in a scope (or telescope or microscope or camera) by the internal structures but those are the "Big 2".

In a scope, they sometimes use a Field Stop so you don't see the edges. Anything that makes the image pass through a space smaller than it actually is will decrease the FoV.