They've only had 1 game decided by 1 goal (not really a goal, a shootout) and 2 games decided by 2. When you are up big or down big, you don't double shift or skip line changes as much. The last 2 games in particular, the Pens have been way ahead, so there's no reason to keep throwing Malkin out there. In those situations you will typically roll 4 lines and keep everyone fresh.
I think the ES time difference between Sid and Malkin is mostly accounted for by face offs. During the important face offs, DB is either putting Sid out there with Malkin or putting Sid out there on another line.