MacBook window getting bigger?

For some time now, I've been having a problem on my MacBook Pro where my windows "increase by themselves." For example, I open Word and Safari and drag the windows so they fit side by side. Then I minimize the Safari tab and open another one, and the Word window expands to its full size (not full screen, it just covers the screen from left to right). The same thing happens when both windows are open and I briefly do something else and then come back; one of them expands again on its own. I can't figure out why this is happening; maybe someone knows…