Wingtip
I think it has to do with wingtip vortex.
In subsonic flight, some of the air underneath the wing escapes around the tip of the wing to create the wingtip vortex. This means loss of lift, though. Therefore, a long but thin and narrow wing is efficient compared to short but thick and wide wing - the ratio of lift to induced drag is better.
Whereas in supersonic flight, the air compressed under the wing cannot escape sideways around the wingtip - not enough time. Therefore a short and wide wing is not less efficient than long and narrow one.