Is it really true that in places with longer jail sentences (specifically, tile spent in prison) and/or higher prison populations as a share of general population that there is less crime?

Just to throw my two cents in. We could look at Singapore which does have harsher sentencing and low crime rates. However, there is a bit of caveat. Singapore has its red light districts where law enforcement sorta turns a blind eye. Its more of a, "so long as you keep your bullshit there, be classy about it and don't let it spill out, we will just conveniently ignore it."