>Agreed. It's interesting to compare with Facebook's web layout. The timeline column isn't much wider, but Facebook fills the extra space with information. (Menu, Trending stories and ads etc, friends' activities.)
Perhaps there's a correlation to my preference to G+'s layout, but one of the first things I did when I signed back up for FB was make a userscript to remove all that noise. My FB now looks like http://i.imgur.com/Vd6e6sh.png
I think it looks so much better single-column, personally (though I use the 3-column layout on G+).
For an experiment I opened up the dev editor in Chrome and changed the CSS of the content blocks/cards so the max-width was ~700px instead of 530px and it looked better. The content expanded just fine in a larger form so I'm sure it could go bigger.
I'm not sure why they chose such a small default width for desktop.
I'm generally in favor of a wider column because then the in-line images are bigger. And in either case, I don't see why the column width shouldn't be adjustable.
Of course, since I stopped using G+ years ago, I have no reason to care....