According to Shanken News Daily, "In 2011, Napa County produced around 20% of all California wines, and accounted for 16%-17.5% of all wines produced in the U.S." Wow. They say Napa County, not Napa Valley so there are some producers included who might otherwise not be when most of us think of Napa, but according to Wikipedia, the difference in acreage between the two is only 2,275 in favor of Napa County. That equates to just over 5% of Napa Valley's planted acres, not a huge amount.
Napa Valley arrives with a sense of exclusivity but those numbers make it sound less boutique and more shopping mall.
It's possible some games with numbers are being played, otherwise why not report Napa Valley alone. Do wineries in NapaValley producing wine from other regions of California count in the Napa County numbers?
At any rate, the mot important thing for me to remember about production size and an air of exclusivity is that Bordeaux boasts three and a half times the planted acreage of Napa Valley! Is that a bubble I hear bursting?