In some ways it is. Especially on portable laptops. I also tend to use smaller monitors because I already have enough clutter on my desk.
But I suppose it isn't the screen space that is the issue, if the UI makes poor use of it, you also have to do a lot more travel to achieve things and it becomes slightly slower as a workflow.
Weirdly it also looks more consumer centric, like a TV or other multimedia appliance rather than a workstation class tool.
I wonder if anyone has ever done a serious usability study against Mac OS X vs Windows NT 4.0 UI systems? I know Sun Microsystems did one on Gnome 2 (a slight modification called Java Desktop System) for Solaris 10 as part of the CDE migration. The results of that did actually compare favourably against Aqua.