When running on Wayland the cursor is too large over most windows (Plasma, title bars, Qt, and Electron apps). The only apps where the cursor is appropriately sized I found are (wayland-native) Firefox and Thunderbird.
See screenshots for difference between X11 and Wayland
Plasma master, Nvidia GPU with 510 driver, 2 Monitors with 200% scaling
Created attachment 146175 [details]
Cursor on X11
Created attachment 146176 [details]
Cursor on Wayland
Setting the size to 24 makes it work as expected, so the behavior kind of makes sense. However there's an issue with the KCM showing the wrong preview size
Cursor size means different things on x11 and wayland. On X11, it's in device pixels. On Wayland, the cursor size is in logical pixels and kwin upscales it as needed. You need to use 24 cursor size on wayland.
The cursor size issue in kcm is already reported https://bugs.kde.org/show_bug.cgi?id=392155.