Opening MS Word documents, keeps filling screen

I use a large monitor and when I open up a MS Word document, it will randomly open it up in almost full screen mode. I only want to use it in the corner of the screen and make the most of the rest of the screen for other apps. There are times when I minimise the window to the size I want it, and it will randomly flip it back to full size.

So far, this only happens on Word and not excel... It really is driving me mad.