Apple has changed the functionality of the zoom button for every window in OS X Yosemite, the third and green button. It’s not a zoom button anymore (nor a maximize as in MS Windows), it is now a fullscreen. Which at first (and second time) is annoying and confusing. There doesn’t seem to be a way to permanently change this back to a just doing the zoom as previous. But you can change it temporarily each time by hitting Alt/Option. This will change the button’s functionality to zoom. Visually it’ll change from fullscreen icon 
(two arrows/squares) to the zoom icon 
(plus icon).