Relevant part:
Huge cursors in GTK4 apps
This problem is already fixed in the main branch of GTK 4, but it’s not backported to 4.16 yet, probably because the fix uses a Wayland feature that Mutter doesn’t support yet. So at the moment, your only option is to use a different cursor theme whose “nominal size” and “image size” are equal.
It‘s coming along in Thunderbird, they continuously mention it in their monthly development blog.