It's not that it's not big. It's that it's not as big as it could be. It's not gmail-big or maps-big, at least for me. It is outclassed by many other, in their respective niches (video streaming, music, and social networking), and despite the huge efforts, Google never managed to turn it into a subscription-based service.

It's not that it's not big, it's that it is not on the same level than maps or gmail, IMO.