Why are buttons icons so small in the Customize window?
This is probably because you are using QAP on an HDPI-devices (like Surface Book). On these screens, icons on QAP Customize window may not scale properly. See this example (taken from QAP v8 but this has not changed since): on the left, the Customize window on a standard desktop PC and, on the right, the same window on a Surface Book.
The scaling effect also impacts icon sizes in QAP popup menu. You could compensate this scaling by changing menu icons size in Options but this would not fix the buttons icon size in the Customize window.
Thanks to QAP user Jörg Giencke, you can use this tip from brianapps (the developer of Sizer 4.0): right-click the Quick Access Popup executable icon in Windows Explorer, select Properties, then Compatibility, check Override high DPI scaling behavior and select System from the dropdown list. This goes for Windows 10 Creators Update (and other Windows 10 version on HDPI-devices?). After that, everything displays just fine on Surface and other HDPI devices.
It is recommended to reboot the computer in order to enforce the change and clear out older settings or window displays.