I've tested this in Internet Explorer 10, Firefox and Chrome and I'm unable to replicate 'The larger font' and the large box around the results, I suspect this is because your either using another browser, a plugin or a customization from being logged in...
In terms of the extra links this varies from site to site and these are refereed to as 'site links' The number of site links varies from site to site and search to search, it is determined on many factors to how important Google believes your site is and how relevant to that of the search. There is a lot of threads on Pro Webmasters already that you should check out, one here > What are the most important things I need to do to encourage Google Sitelinks?
I see nothing strange or special treatment going on here.... Webmasters Stack Exchange will be in all bold that is your search term and its a 100% match (Title, Domain Name). However I am unable to explain that box - through just tested on my laptop and again I do not see what your seeing and believe this is some kind of caching, or something else either your Browsers end or Googles.