Google Operating System blog has screen shots of Google showing larger thumbnail images for videos that come up in the search results.
The larger video thumbnails show up for the first video result on tablet devices.
I personally tried to replicate this on my devices, including Chrome and mobile Safari on an iPad and some Android devices, and I only saw the smaller thumbnails.
As you can see, the video thumbnail is larger, the +1 button was added, and the snippet (description) in the search results was removed.
Google is constantly testing search results user interfaces, so this should come as no surprise.
For more tests, see our Google User Interface category.