Cursor Size Problems In Wayland, Explained
Quoting: Cursor Size Problems In Wayland, Explained - KDE Blogs —
Unlike Adwaita, the image size is larger than the nominal size. That, combined with a global scale, triggers the bug in GTK4. Explanation of the bug.
XCursor allows the image size to be different from the nominal size. I don't know why it was designed this way, but my guess is so you can crop the empty part of the image. This both reduces file size, and reduces flicking when the cursor changes (with software cursors under X11). But the image size can also be larger than the nominal size, and Breeze (and a lot of other themes) uses this feature.
You can see in the above images that the "arrow" of nominal size 24 in Breeze is actually similar in size to the same nominal size in Adwaita. But the "badge" in Breeze is further apart, so it can't fit into a 24x24 image. That's why Breeze is built this way. In a sense, "nominal size" is similar to how "font size" works, where it resembles the "main part" of a character in the font, but some characters can have "extra parts" that go through the ceiling or floor.