A factor is that countries like Germany don't imprison first time criminals, but give them probation mostly. And sentences are much, much shorter than in the U.S.

Well, for instance[1] shows that the violent crime rate has been 2.5 to 3.5 times as large as in Canada between 1980 and 2000. That still leaves a factor of 2 to be explained. The other statistics in this report only makes the difference in incarceration rate harder to explain. The most common explanation I have heard is that in the US, people that are accused are both prosecuted and convicted more often. It seems extremely doubtful that the US police force and justice system could be twice as effective as that of other Western European countries.

[1]http://dsp-psd.pwgsc.gc.ca/Collection-R/Statcan/85-002.../01...

I'm more than willing to let that factor of 2 come down to a fubared justice system--or possibly to longer prison sentences, which is possibly a "works as designed" type of bug.

Big complex problems have big and complex sets of causes. Higher actual crime is not an outlandish partial cause of a higher prison population, even if it isn't a complete explanation.