The big issue here is that both sidebars have a fixed size, and that the middle content area is dynamically resized, which basically means that the wider your browser window is, the larger the content area is.
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As for now lets take a look at some of the solutions for this issues.
Fix Google+ Whitespace
♦ Whitespace Remover for Google Plus
If you'd like to cut down on that whitespace and center everything, Whitespace Remover for Google Plus is a Chrome and Firefox extension that does just that.
Whitespace Remover doesn't overhaul or change the layout much. By default, Google+ has the navigation bar and stream on the left and leaves a ton of whitespace on the right. Whitespace Remover centers everything and adds a grey box around it all. Doing so cuts down on the brightness and makes it easier to look at on a widescreen monitor. So is NOT a whitespace "remover," rather just a whitespace "mover" - still just as much whitespace as before, just the feed is in the center. Liking G-Extend much better.
Whitespace Remover for Google Plus for Chrome (The extension has been removed from Chrome ! Try Balanced Google+ which is a good alternative )and Firefox .