I see all looks good. I don't see any reason to make any changes and the idea to move it to the right side is not good. So it's better to leave it like it is now. The forum is centered and it's wide.  So let's just stop here.

This comes from your WordPress Theme page design, it set 700px for width. 

Try this CSS code, navigate to Dashboard > Forums > Settings > Styles admin page, put it in Custom CSS Code textarea. 

#main article .vce-sid-none .vce-page .entry-content, .vce-sid-none .vce-page .entry-title-page, .error404 .entry-content {
   width: 100% !important;
   max-width: 100% !important;
}

Please delete all caches, press CTRL+F5 (twice) on frontend before checking.  

Hi srtalop,

Please remove the code, I've provided in the previous post and add this one. 

.vce-sid-none .vce-page .entry-content, .vce-sid-none .vce-page .entry-title-page, .error404 .entry-content {
    width: 100% !important;
    max-width: 100% !important;
}

Please watch the video to understand where does this problem come from:

http://www.screencast.com/t/DyQgF3fAh