Every prisoner in the US is a convicted criminal under US law. The amount of criminals outside of US prisons could then be speculated to be much higher than in other countries because the US criminalizes more things than other countries. Determining the number of criminals outside of prison then becomes a hard task since you can't use the state supplied numbers if you want to do a fair comparison between countries. It could also be that an equal amount of things are criminalized in other countries, but not punishable by prison sentence.

Note however that this is wild speculation, just things you have to take into account.