NEW YORK — While big-city jails get most of the attention, lockups in small and medium-sized counties have actually driven the overall explosion in the U.S. inmate population, according to a new analysis of 45 years of jail statistics. U.S. jails now hold nearly 700,000 inmates on any given day, up from 157,000 in 1970, and the Vera Institute of Justice found that smaller counties now hold 44 percent of the overall total, up from just 28 percent in 1978. Jail populations in mid-sized counties with populations of 250,000 to 1 million residents grew by four times and small-sized counties with 250,000 residents or less grew by nearly seven times, Vera's analysis shows.