Let's assume that the fraction of people in prison has grown over the last 70 years. This is either because sentencing has got harsher or because people are committing more crimes. Anecdotally, though, sentencing seems to have got less harsh -- for instance in the 1950s the Boggs act resulted in minimum mandatory sentences of 5-20 years for a first drug offense. Thus, we must conclude that people are committing more crimes than before.

Now, either they're committing more crimes because sentencing has got less harsh, or they're committing more crimes because of other social factors (eg the growth of gang culture, drug culture, et cetera). Either way it's not clear to me that making sentencing even less harsh than it already is will solve anything.