Juries are hardly responsible for murderers spending an average of six years in prison, even if that were true. Sentencing guidelines are a matter of state and federal laws, and until recently those had wide latitude, which the judge was free to exploit. A lot did.
As far as "jury nullification", there have been perhaps two or three such cases of that right being asserted in the last century. It is shot down by virtually every trial judge, and simply saying it will usually get you thrown off a jury (a cute tactic I use when I don't want to do jury duty).