A different way of looking at this issue was pointed out in a recent, knowledgeable letter to the media.
Apparently, we jail fewer offenders per 1,000 serious crimes than virtually any other country - certainly in Europe.
Ergo, our jails contain more prisoners because we have considerably more crime. Remember to combine this to the more generalised effects of liberal sentencing policies which actually increase prison numbers.
More than one in eight in our prisons is a foreign national. Deportation would seem to be a means of remedying recidivism rates amongst these groups.
[If I can track down the statistics, I shall add them to this posting.]
***UPDATE: I now have the figures for Spain. We jail 12 people per 1,000 crimes - Spain just FOUR times that number. QED. Thanks to journalist Leo McKinstry for this information in the Daily Express 3/9/07.