Google is expanding its practice of serving a similar-sized display ad in a larger ad format in order to increase competition for AdSense ad space. Google now allows the 336×280 ad unit to serve 300×250 ads.
Google first allowed smaller ad creative to run in larger ad units with the introduction of the 300×600 and the 970×90 ad sizes to AdSense earlier this year. Google said it would allow 160×600 ads to display in the 300×600 unit and 728×90 ads to run in the 970×90 unit due to the lack of available creative in the new sizes.
Publishers can opt out of having similar-sized display ads served in ad units from the Allow & Block Ads tab in your account. Note that opting out applies at the account level rather than on an individual ad unit.