Google just pushed some updates to its ad serving process enabling the larger ad units to serve similar-sized display ads within.
With this update, Google's ad unit 336x280 would now "occasionally" serve a 300x250 ad when that unit is opted in to showing both the "text and image ads."
"300x250 and 336x280 image ads will both compete for your 336x280 ad unit with the most competitive ad served thus maximizing the earnings on your ad unit," informs Google.
Aside from ensuring that you're getting the most from your ad units, this change gives advertisers the opportunity to expand their reach with their existing ad creatives. "This will maximize the performance of that ad unit for the publisher."
As you can see in the illustration below, when a 300x250 image is served it will be centered in the 336x280 ad unit:
To maximise your earnings potential with this change, don't forget to enable both text and image ads for your ad units.
Additonally, publishers can "opt out" of having similar-sized display ads served in your ad units but remember this may impact your earnings. When you opt out, "this applies on an account level and not just on an individual ad unit," Google expalins.
To opt out, go to the Allow & block ads tab in your account, click the Ad serving tab in the top horizontal bar and select the block option under the "Similar Sized Display Ads" header.
Google had already made this update available to "300x600" and "970x90" ad units.