And Quantcast* disagrees pretty strongly: https://www.quantcast.com/google.com vs
https://www.quantcast.com/yahoo.com. This is only traffic to yahoo.com and google.com, though, leaving out youtube, flickr, and various other domains, but it's likely that's what comscore is measuring on the google side. Given that Youtube's reach is nearly as high as Google.com's, that's probably not helping Yahoo.
* Disclaimer, I work there