Recently Adsense team introduced a feature to customize fonts for the ads now they introduced new expandable ads for Adsense sites. Expandable ads are the rich media ads that can expand beyond the original size of the ad. Expandable ads can stream a movie trailer, show video game clips or display various views of an item for sale.
You will automatically receive expandable ads if you’ve added the Adsense code into your site. You need to enable image ads to receive expandable ads.
Currently expandable ads are only available to specific advertisers located in the U.S. Expandable ads will be served as third-party ads.
You can read more about expandable ads on the Official Adsense Blog here