What defines the "size" of a company in this context is the number of users/buyers, not the number of listed companies. Thumbtack may have half as many pieces of company info, but I bet the number of users that go to Thumbtack for info is tiny compared to Yelp.
I once read that the Yahoo search ad platform has more advertisers than Google, due to more lax standards. Does that mean that Yahoo ads are bigger than AdWords?
For people in the events/ticketing space, this is a big deal. Zvents doesn't get a ton of press, but they are a major player in how people find long tail events on the web, thanks to the fact they power the local section for hundreds of major websites, particularly newspapers.
Right now, StubHub only has bigger events held at "real" venues, whereas Zvents has an index of long tail events that's perhaps 10x bigger than StubHub's. The thing is, StubHub is a secondary marketplace for tickets, and there's basically no secondary market for these long tail events. So does this mean that ebay/StubHub wants to stop limiting themselves to the secondary market?
(Exaggerating, but you get the point...)