How to fix icons and text in programs that suddenly enlarged, on Windows 8.1?
To change the size of your Menus like you've shown, Goto:
Control Panel\Appearance and Personalization\Display
Check the Let me choose one scaling.... option. Under Change only the text size select Menus from the dropdown list and choose your new size (9 is default). Similarly you can select Icons and change them (9 is default).
Personal experience:
"Sudden" size change of desktop items (and web browser content) comes from
holding CTRL and rolling the mouse wheel.