Mind you, text-editing wasn't the designed-for use-case; document-editing was. Open up Pages.app, if you have it, and create a new blank document. The zoom button will set the window to an optimal size for working on the (fixed-ratio) document page. Same for a canvas in Photoshop.

Amusingly enough, the only things it doesn't really work well for are things that reflow their content arbitrarily to fill the window: text, HTML, and terminals. The three things developers spend all their time looking at!