The history of the UX of the arduino editor, came from wiring, which came from processing, which came from Design By Numbers ( http://dbn.media.mit.edu/whatisdbn.html ).

One of the surviving ideas was that this is an editor for beginners, to the point of having a smaller window was less intimidating, that it made the code seem like a sketch, and more encouraged playful experimentation.

This is my short, incomplete understanding of why the window is small when you open arduino/processing.