Really?
Only about 18% of current prisoners (state and federal) are serving time for drug offenses. [1]
Of federal prisoners serving time for drug offenses, only 12% are primarily about marijuana. (54% are cocaine and 24% are meth.) [2]
Only 14% of federal offenders were subject to a mandatory minimum sentence. (About half of those were drug offenses, so this overlaps heavily with the 18% figure above.) [3]
No realistic minor changes could reduce the prison system to 1/8 of its size. 51% of prisoners are serving time for violent offenses. In fact, 14% of prisoners are serving time for homicide alone, and a similar number for rape. [4] So if you decriminalized _every_ crime except homicide and rape, and cut the sentences for homicide and rape in _half_, then the prison system would be 1/8 of its current size.
I am optimistic that the USA could eventually, in the very long term, reduce the prison system to 1/8 of its size. Fifty years ago, the prison system was 1/4 its current size. [5] Most western European countries have between 1/8 and 1/4 the US incarceration rate, and a few have below 1/8. [6] But this will require way, way, way bigger societal changes than just marijuana decriminalization or other minor tweaks.
[1] Source is https://www.bjs.gov/index.cfm?ty=pbdetail&iid=6846. Of the 1.274M state prisoners, 14% are serving time for drug offenses. Of the 162k federal prisoners, 47% are serving time for drug offenses.
[2] Source is https://www.bjs.gov/content/pub/pdf/dofp12.pdf. Caveat: this is federal-only, and federal prison statistics are often different from state prison statistics.
[3] Source is https://www.ussc.gov/sites/default/files/pdf/research-and-pu.... Caveat: this is federal-only, and federal prison statistics are often different from state prison statistics.
[4] Source is https://www.bjs.gov/index.cfm?ty=pbdetail&iid=6846 again. Of the 1.274M state prisoners, 56% are serving time for violent offenses (16% for homicide). Of the 162k federal prisoners, 8% are serving time for violent offenses (2% for homicide).
[5] https://en.wikipedia.org/wiki/Incarceration_in_the_United_St...
[6] https://www.statista.com/statistics/957501/incarceration-rat...