From the Pew Research Center [emphasis added]:
President Barack Obama is on pace to leave the White House with a smaller federal prison population than when he took office – a distinction no president since Jimmy Carter has had, according to a Pew Research Center analysis of data from the Bureau of Justice Statistics.
The number of sentenced prisoners in federal custody fell 5% (or 7,981 inmates) between the end of 2009, Obama’s first year in office, and 2015, the most recent year for which BJS has final, end-of-year statistics. Preliminary figures for 2016 show the decline continued during Obama’s last full year in office and that the overall reduction during his tenure will likely exceed 5%.
By contrast, the federal prison population increased significantly under every other president since 1981.
During the eight years of Ronald Reagan’s tenure, for example, the number of sentenced inmates in federal custody rose 78% (or 16,539 prisoners), the largest percentage increase for any administration on record. It went up 39% (16,946 prisoners) during the four years of George H.W. Bush’s presidency; 56% (38,769 prisoners) during the eight years of Bill Clinton’s tenure; and 32% (36,784 prisoners) during the eight years of George W. Bush’s administration. In absolute numbers, the increase during Clinton’s tenure was the largest on record.