The size of the cursor when "Resolution dependent cursor size" is meant to keep the size of the cursor (perceived by the user) the same over different screens with different resolution. Though, it would be useful to allow the user to increase of decrease the size of the cursor, so that it scales in a relative way on all the screens. When I select this option the cursor is way too big. I think that the cursor size should be DPI dependent as mention in Bug #377559. Also in that case it should be possible to configure the size of the cursor.
With wayland scaling this option is removed as it makes no sense and now (5.14) scales to the screen appropriately. On X this is not doable.