There are two threads at WebmasterWorld in Yahoo! Publisher Network forum that discuss the same topic, they are named; YPN Testing Stretched Ads and the other is named Why do I get just one ad on my wide skyscraper.
First, Yahoo! Publisher Network has been doing this as far back as August 19th of this year. We have an entry on it with screen captures named YPN Randomizing Number of Ads.
The members in the threads are complaining about how Google increases the ad font of the ads when they use a less ads in the wide sky scraper banner. But I think that Google only does that for site targeted ads (I may be wrong and I'll ping JenSense to confirm) but I took some screen captures this morning of Google's non site targeted ads on this site and guess what? They look like YPN sized ads. See for yourself...
I believe the only time an ad fills the ad box is when its a site targeted ad. I can not bring up an example of an ad on my site at this time. But there are often examples of them on the homepage of the blog.