Big-city jails grab headlines but a new report shows that small and medium-sized counties have driven the overall explosion in jail growth rates.

There are now nearly 700,000 people locked up in local jails in the country's roughly 3,000 counties on any given day, up from 157,000 in 1970.

Forty-five years of jail statistics analyzed by the Vera Institute of Justice show smaller counties now account for about 44 percent of all jailed people in the U.S. That's up from just 28 percent in 1978.

Exactly what's behind that trend is not clear. But experts say a range of factors likely contribute, from law enforcement's increased use of summonses and traffic tickets to the closing of state mental hospitals.