We've a new Adsense setting; Similar Sized Display Ads - Show smaller but higher performing display ads in larger ad units. Also they've moved settings to new Ad serving tab, next to Ad review center.
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.