bogner - This isn't something under your control. Google has been experimenting with serp displays for some time, and Sitelink displays have been constantly recalibrated based on Google user reaction to them.
In
Google's December Search Quality Highlights [insidesearch.blogspot.com], Google noted that they'd made changes to show "more relevant sitelinks", and it's likely that the timing of the changes you saw coincided with the algorithmic change.
We discussed Mega Sitelinks here in Aug 2011, at the time they were launched....
Mega Sitelinks - everybody can see big sitelinks now http://www.webmasterworld.com/google/4349306.htm [webmasterworld.com]
I've come to feel that the additional exposure Sitelinks provide has been used to test searcher engagement on a site, which in turn has allowed Google to adjust both its rankings and it interface. As I posted in the above discussion...
I'm guessing that measures of user engagement will allow Google to calibrate the new sitelink algo even more. I also feel that these sitelinks go hand in hand with Panda. They should provide some hints about how Google is trying to improve engagement on its own pages.
It appears that Mega Sitelinks are now shown only for the largest sites. It's likely that for Mega Sitelinks to be justified, there's a combination of site size, channeled content, and user engagement within those channels, which Google has found to be a good threshold value.
The Mega Sitelinks have also gotten much broader... ie, less specific. In the Aug 2011 discussion, I noted that a local weekly newspaper site was returning 12 Mega Sitelinks, one of which was one of its most historically popular stories.
The same site is now returning only 6 Mega Sitelinks, and the categories returned are: Music, Arts & Culture, Food & Drink, Movies, Contact Us, and Classifieds.
Also see WebmasterWorld discussion on Google's December Search Quality Highlights [webmasterworld.com...]
