David,
The way you describe your thinking is almost correct except you need to think about it in terms of the object size on the film. Forget about angle of view for this. For a 300mm lens to give the same object size on film as a 50mm lens, regardless of film format, the 300mm lens needs to be further away from the subject to compensate for the additional magnification of the 300mm lens. It is the altering of distance from the subject which alters the perspective and relative sizes of different parts of the subject at different distances from the lens.
This is very well illustrated in AA's "The Negative" in the chapter on "artificial light photography". There is an image of two pianists (fig: 7-14 in my edition) and a longer lens was used to keep the relative sizes of the two pianists fairly equal on film. Had a shorter lens been used the camera would need to be closer to the subjects thereby making the nearer pianist look much bigger than the more distant pianist.
So the selection of lense focal length is a very important tool in adjusting the relative importance of different parts of the subject according to your viewpoint.
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