As you’re probably aware, Apple changed the functionality of a green button click in Yosemite: it not longer “zooms to fit” (which could have different behaviors per app), but instead it enters Full Screen mode (if the app has one; otherwise, it does zoom to fit). If you want zoom-to-click, you can option-click the green button; there’s no apparent way to toggle these two functionalities.
But today, I discovered a second zoom-to-fit shortcut: double-click the window’s title bar. This is easier to do, as you don’t have to hit the small green button target; just get the mouse anywhere in the title bar, and double-click.
Note that this won’t work if you have “Double-click a window’s title bar to minimize” enabled in the Dock System Preferences panel.