Your shortcut to make text larger is shown to be Command-colon instead of Command-plus.
Maybe you added it yourself in System Settings->Keyboard?
Go to System Settings->Keyboard, click "Keyboard Shortcuts", the "App Shortcuts" and see if there one for
"Bigger".
Also: what is your keyboard layout? Different layouts may sometimes cause shortcuts to differ.
And in many layouts, you'd also need to use the Shift key to produce a colon; hence I am thinking maybe you are using French layout? That one produces a colon without requiring a modifier.
For example, in the US layout you'd be seeing Command-Shift-semicolon, not Command-Colon.