https://support.apple.com/kb/PH25079
You might've maximized the window by double clicking on the title bar of the app. This is a preference that can be set in the
Dock System Preference. "Double-click a window's title bar to: zoom"
In any case, try double clicking the title bar again to undo the zoom. This is different than what the green maximize button does.
Here's an example, although the Mac appears to be displaying Japanese(?), it shows the two options for double-clicking on the title bar. One is maximize, the other is minimize.