The two biggest reasons they found? Communication and defect rates.
This is an interesting debate for me because some of the feature teams we have at Hotmail would consume all the developers of some of the small companies mentioned, but we also have PMs and testers who are on point to help handle some of the inefficiencies in communication and catch defects as they are coded. I imagine some of these smaller companies do not.
All in all, I've seen benefits firsthand to both approaches.