Telephoto "Compression" question

Hello all - I know that this forum quite often talks of equivalence and I've got my head around most of it - BUT I've just been trying to work out if telephoto compression is caused by the length of lens or angle of the field of view.  I think it's FOV - but I don't have any teles in m4/3 to test yet - can someone with more experience of long lenses on a smaller format let me know?

Thank you!

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James
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