What you want IINA to do:
Now that mpv supports MKV crop metadata with v0.37.0, IINA should support it too and resize the window accordingly

What IINA does currently:
IINA currently ignores MKV crop metadata and displays the full video size instead.

Why you think this should be added:
Cropping without re-encoding is a very useful feature of the Matroska container — especially for cinemascope movies with black bars encoded into them — and more players should support it (as does mpv now)!

Examples of other projects that have something similar:
mpv v0.37.0