I am using an iMac and when I view my profile, the page doesn't fit the Safari window. A small portion of the page in the bottom stays white. The height of the page could be made responsive so that it completely fits the window.
Looking at your screen shot I see that the problem is not that your resolution is too low, but rather it's too high. This question is about what happens to the extra pixels. (Reader: if you can't tell, there is a swath of white space below the grey bar; that's what we're talking about.
From what I can see, this is not iMac (or Mac, or browser)-specific; that's just how the site is laid out. If any user makes their font size small enough, they'll see the same thing. The footer is not floated to the bottom of the screen; it's just at the end of the content.
That said, I have to make my font tiny indeed to make it look that way on my (1920x1200) screen. I don't think most people will have seen the layout rendered this way.
You mention that you're using ctrl-+ (or, on your Mac, cmd-+) to zoom in to get rid of the whitespace. This suggests to me that, being familiar with this browser zoom feature, you may have used it previously and be at a non-standard zoom level.
ctrl-0 or cmd-0 will set your browser zoom to the default level. If this fixes the problem, you're browsing in a zoomed-out format, so the site's existing responsive layout will not work.
I suppose that some effort could be made to make sure that the page works perfectly at all zoom levels, but I'm not sure that it would be worthwhile.
**Update: Screen shot from OP after cmd-0 here still shows the problem.