Sure most Mac users don't run full-screen but no Macs from the past few years have native displays limited to under 1024 pixels wide. Even without maximizing, most people (in my experience with running stats on this stuff) run at > 1024. Without pulling out my data, I found a browser width of around 1100-1150 pixels to make up the lion's share of my users.
This website gives the impression that she still needs some practice in her UX design skills.
The maximize button for browsers on Mac's maximizes the browser window to fit the content within the page. That is exactly how I want it to work when working with higher resolutions. Never do I want to maximize my browser to my 1920x1080 resolution.
I could see that while that solution is great for larger resolutions it may be non-optimal for smaller one's so maybe adding a system wide toggle switch would be ideal.
(I am not associated with this app, just a happy customer.)
Both can be gotten through Irradiated Software (http://irradiatedsoftware.com/). Free demo.
I'm just a happy customer too.