That's a result of copy/paste content in quoted post. I see <div> tags with "content" class, and this class is the main class of your theme with 1200px width. wpForo quotes itself work fine without any issue. You've quoted a div with 1200px width so you get this issue.

I recommend put this CSS code in Dashboard > Forums > Settings > Styles > Custom CSS Code textarea. Save it, delete all caches and press Ctrl+F5 on forum front-end:

#wpforo #wpforo-wrap .wpforo-post-content .rh-container, 
#wpforo #wpforo-wrap .wpforo-post-content .content {
      width: auto;
      clear: right;
}

That fixed the issue.  Thanks.