The Savage Progression Duergar (scroll down) gets
enlarge person as a racial SLA. Most notable is that this ability affects "only the [Duergar] and whatever she carries."
You can carry creatures, such in your hands or your pack. If
enlarge person would affect you, and the Duergar carries you, then you should be enlarged too.
This silly literalist reading is part of the title and the concept, but what do you think?