My MacBook's external monitor is magnified
Maybe you accidentally magnified your screen using Universal Access's screen Zoom, it can survive display detects with out zooming back out. And it can be accidentially triggered via holding down the &#8984; key or some customizable key and then scrolling up or down with your mouse or track pad. 


Look in System Preferences under the "Univeral Access" section.  
Once there click on the "Seeing" tab. Is "Zoom:" on? If yes then continue. 
Press &#8984; + &#8997; + - to zoom out.