The big crime-policy news today: The Justice Department will no longer pursue steep mandatory-minimum prison sentences for many low-level, nonviolent drug offenders who aren't affiliated with gangs or larger organizations.

Why the shift? In his press conference, Attorney General Eric Holder argued that many of America's incarceration policies are inhumane and counterproductive. “We must face the reality that, as it stands, our system is, in too many ways, broken,” Holder said. "Many aspects of our criminal justice system may actually exacerbate these problems rather than alleviate them.”

But here's another way to think about today's move: The rapid growth in federal prisons was putting a serious strain on the Justice Department's budget. The number of federal inmates has grown tenfold since 1980 and now surpasses 218,000. Housing all those prisoners isn’t cheap: The average minimum-security inmate now costs $21,000 a year, while the average high-security inmate costs $33,000 a year.

Add it all up, and the Obama administration had to request $6.9 billion for the Bureau of Prisons in fiscal 2013. That may not sound like much in the context of trillion-dollar deficits. But a recent report (pdf) from the Urban Institute pointed out that if the current rate of incarceration continues, federal prisons will keep taking up a bigger and bigger chunk of the Department of Justice’s budget — rising to 30 percent by 2020:

That could create real problems. Since Congress is trying to rein in overall spending, prisons could end up crowding out a bunch of other key Justice Department priorities, such as federal investigators or support for state and local governments. Many of those latter programs, the Urban Institute argues, do far more for public safety than prisons.

So how do drugs fit in here? As the report notes, about 32 percent of the growth in the federal prison population has come from longer prison sentences for drug offenders. The report notes that Congress would likely need to change sentencing guidelines if it really wanted to curtail the growth of federal prisons.

Holder's announcement today is a small move in that direction (and some members of Congress, like Sen. Rand Paul (R-Ky.), have proposed further steps).

To be sure, this move won't, on its own, shrink America's total prison population all that dramatically. As my colleague Dylan Matthews points out, those 219,000 federal inmates make up just 13.9 percent of all U.S. prisoners — the rest are held at the state level. Really large-scale reform would require new policies from state legislatures around the country. (Update: Here's a new report describing how 17 states are carrying out exactly these sorts of reforms.)

But that doesn't mean this move is negligible, either. From the Justice Department's perspective, the rapid growth of federal prisons was quickly becoming unsustainable.

* Correction: The federal prison population has grown tenfold since 1980, not quadrupled as originally stated.