https://i.pinimg.com/474x/f1/9b/17/f19b17967661a5b78f6213e6a4f0baa7.jpg

Some ears are well-suited for oval shapes (and I assume this is currently more cost-effective for machining in manufacturing).

But more often than not, ears match better with a D-shape.

I think it’s just normalized for over-ear cups to often be partially-on-ear cups. Some cups fit over my ears (rectangle ones for me), and I greatly prefer them.

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(Edit: I wrote this before I saw the other comments about how the cup shape affects the speakers’ ability to transmit into the ear.

Presumably circle, oval, rectangle, and D-shaped cup designs require specific attention to the design for the speakers.)