In the iTunes preferences you can set the default font size to "Large" for the source lists. You can also use the "Zoom" option in the Window menu to fill the window for the whole screen.
In general, though, few Mac applications are designed to fill the screen like Windows applications, though. It's a difference in the "philosophy" of how they treat windows. Since the menu on the top is always contextual to the foreground application, there's no need to fill out the whole window. That way you can tile your windows to easily switch back and forth, or keep an eye on something while doing something else. For example, I now have Safari open on one side of my screen, while I watch a movie in the other part.
Welcome to the Mac!