Yeah, got no idea on this one. Two “icon themes” appear to retain the correct scalable icon size: high-contrast and moka. The new desktop icon appears to be the problematic one.
Reboot, remove and re-add the shortcut from the menu, trying to “resize” the icon produces no change in the size.
Maybe a .svg issue that may have been included in all the reziable icon folders in /usr/share/app-install/icons
…I otherwise have no idea.