Suddenly More Space on My Comcast DVR?

Two days ago, the amount of space used on my Comcast DVR (Motorola 3412) dropped from 75% to 44%. I still have all of the same shows on my DVR that I had before. Nothing was erased. Does anyone know why? Did Comcast initiate a new compression scheme that even affects previously recorded shows? Perhaps temporary data was cleaned up and it freed space?