From a report by the National Association of Assistant United States Attorneys, “The Dangerous Myths of Drug Sentencing ‘Reform,’ ” July 2015 (footnotes removed):

While drug offenders accounted for a large percentage of the federal prison population in 2013 and 2014, virtually all of those were convicted of major drug trafficking crimes, including the trafficking and sale of significant amounts of dangerous drugs like heroin, methamphetamine, cocaine, crack cocaine, and oxycodone. Despite claims and insinuations to the contrary, only a tiny fraction of defendants convicted in the federal court system were convicted of possession of drugs for personal use. U.S. Sentencing Commission data reveals that only 0.9 percent of defendants sentenced for drug offenses in federal court in 2014 were sentenced for simple drug possession and many of those charges were the result of plea bargaining.

It is significant too that the percentage of federal prisoners serving sentences for drug trafficking is higher than for state prisoners. This stems from the somewhat obvious fact that federal criminal jurisdiction over murders, rapes, and robberies (crimes that are largely intrastate), is limited and those crimes are, for the most part, prosecuted in state courts. Large scale drug trafficking (activity that is inherently interstate and international in nature) is proportionately more often investigated and prosecuted in the federal system. As a result, the federal system focuses on, and the federal prisons house, many of the most dangerous and serious drug traffickers—offenders that are often violent, repeat offenders involved in high level, large scale trafficking.