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To help increase competition for some ad sizes, we occasionally show similar-sized display ads within larger ad units. When this happens, the smaller ad is served neatly in the center of the larger ad unit. For example, you might see the following:
a 160x600 ad in a 300x600 ad unit
a 728x90 ad in a 970x90 ad unit
a 300x250 ad in a 336x280 ad unit.
Note that this is not the complete list of ad units that may show smaller ads and the list is subject to change.
We’ve already updated this for our 300x600 and 970x90 ad units and now we’ve extended this flexibility to our 336x280 ad unit, allowing it to serve a 300x250 ad when doing so will maximize the performance of that ad unit for the publisher.
With this change you may now occasionally see a 300x250 ad served in your 336x280 ad unit when that unit is opted in to showing 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. When a 300x250 image is served it will be centered in the 336x280 ad unit...