Icons aren't the right size (don't scale at larger sizes, or some are big)

Hi, under plasma5 (Kubuntu 18.04.2 LTS, default here), with Preferences→Interfaces→Theme→Change icon theme: set to Use system icon (default option), the icon for the "Preferences" and "About Inkscape" appears in very big, deforming all the graphic user interface. Workaround: switching the theme to "Tango". Here is a series of screenshot to better show the issue: