Not really a support question, just idle curiosity, so I thought i’d post it here.
When running Plasma in Wayland why is the cursor large on the desktop
but normal size in a app window. Is it suppose to be like that ?



Wayland is still very much work-in-progress.
Things are still missing, and not everything is working correctly.

See e.g. https://community.kde.org/Plasma/5.12_Errata

This particular problem is probably a bug in kwin, although it might also be a problem in Qt itself.
You may try if setting a specific mouse cursor size instead of automatic helps.

I do remember a bug report about this problem (or read something about it somewhere else), but I’m not sure I will find it.

PS, there is a bug report here: