Comment 20 for bug 1304067

I just installed a fresh copy of Xubuntu 18.04 on a new system with a 4k screen, and I am having the same issue.

When I boot and login the mouse is huge when on the desktop, but changes sizes when hovering over certain applications. As was mentioned by Manuel, if I change the cursor size setting or simply change the cursor theme from "Default" to "DMZ (Black)" and then back to "Default", the problem disappears.
 The size is normal and constant no matter what application I hover over.

I found this older post from someone who was using Xubuntu 14.04 a few years ago, and I believe it to be the same bug. The images and behavior accurately describe what we are seeing in 18.04.

https://superuser.com/questions/768830/mouse-cursor-scaling-issues-in-xubuntu-14-04-with-xfce-4-10

I'm also wondering if the issue is somehow related to this bug:

https://bugzilla.xfce.org/show_bug.cgi?id=7415

In that instance I see no mention of size changes, but they similarly have changes in the applied cursor theme based on whether the mouse is hovering over the desktop or other applications.