STR:
Check Preferences -> MISC. -> Minimize to system tray.
Minimize NPP.

Result:
The 32x32 image is re-scaled to 16x16.

Notepad++ v7.9 (32-bit)
Build time : Oct 4 2020 - 23:15:47
Path : C:---\notepad++.exe
Admin mode : ON
Local Conf mode : ON
OS Name : Windows 7 Enterprise (32-bit)
OS Build : 7601.23964
Current ANSI codepage : 1255
Plugins : ComparePlus.dll HTMLTag_unicode.dll MenuIcons.dll PythonScript.dll _CustomizeToolbar.dll