Why does the Save Icon keep getting tiny and then reverts to normal size?

I am running LibreOffice Version: 6.3.2.2 on Windows 10 Home version 1903 Build 18362.356. No matter what Icon style I use in Calc, when I save a spreadsheet, the Save Icon gets tiny. Later when I reopen it is back to normal.

Is that a bug or a problem with my setup?

Try to disable OpenGL in Tools -> Options -> LibreOffice -> View -> Category: Graphics Output -> Option: Use OpenGL for all rendering, if set.

tdf#126293 - by now fixed only for 6.4.