To be fair, the US doesn't imprisons a larger percentage of its population, the US imprisons a larger percentage of its criminals.Which is absolute bullshit, as the United States imprisons a larger percentage of its population than any other developed liberal democracy in the world. The U.S. is far less lenient than those countries that have stricter gun control laws and less violent crime
Also the sentences served tend to be longer than in European countries so the convicts tend to "pile up" behind bars. Half of murder/manslaughter convictions get 25 years to life. 90 percent convictions for robbery, burglary and drug offenses receive 10 years or more. In the case of drug offenses, there are drug laws in America that simply don't exist in many European nations.