According to the Bureau of Justice Statistics:
Sixteen percent of state prisoners were serving
sentences for drug-related offenses (206,300 prisoners).
Almost 50% (92,000 prisoners) of sentenced federal prisoners on
September 30, 2015 (the most recent date for which federal offense
data are available) were serving time for drug offenses
Doing the math on that, about 30.8% of drug prisoners are in federal prison.
Someone pointed out that the stats on possession vs all drug offenses may be different. And that turned out to be the case, in a large way.
According to this report, (see page 30) 46,000 of those 206,300 state drug prisoners had drug possession as their most serious charge.
But according to this report, (table on page 2) only 247 federal prisoners had drug possession as their most serious offense in 2012.
Doing the math, only about 0.5% of prisoners convicted of drug possession as their most serious crime were in federal prison as opposed to state prison.
It's worth noting that some people may be in prison for more than one reason, so giving someone a pardon for their drug offenses may or may not result in them actually being released.