Why do some posts have 'big' pictures, and others do not? I emailed Steve Totilo, and he suggested that...

If you are an admin, you can not only 'promote' articles and posts to the top of the TAY, but you 'liking' any article (apparently) guarantees it being made into having a 'big' picture as a thumbnail/preview.

Therefore I'd be careful about which articles you're liking.