The buttons that made windows bigger/smaller are not quite that. The button toggles between the natural (smallest) size of a window that is sufficient to display all the contents and the last size other than minimal. It was very convenient for document windows. E.g. you have a spreadsheet with a few rows and columns and lots of white space: click that button and the window shrinks to display only the contents, leaving the empty cells hidden. It's also good for Finder windows that have only a few icons.

If you hold down the Option/Alt key, it will try to resize to the largest possible size, similar to maximize.