Yes, up until now the green button stood for "resize the window to make the best use of available space". It was up to the app to figure out what that meant. For example, it might make the window just large enough to look at the document or image without the need for scroll bars, or it might make the window tall enough to take up the entire vertical width of the window, and just wide enough to fit the document without the need for horizontal scroll bars.
The problem is that each app developer has a different idea of what "best use of space" means, so it was inconsistent. And pretty much everyone who came in from the Windows world expected it to fill up the entire screen, not "take as much space as you need".
