While in a Nephalem Rift, you have a progress bar which shows your completed percentage of the rift - when the bar fills to 100%, a Rift Guardian will spawn. Occasionally when I kill a monster, the percentage of my bar does not increase.
Because of this, I'm led to believe that some monsters grant a larger amount of progress towards the completion of the Rift than others.
Is there any documentation or are there any statistics which show the specific percentage gains each monster gives in a Rift? Also, do I receive any bonus gains in percentage for massacre (or other types) bonuses?