Your plant looks great. I'm envious of your live sphagnum. I've got a couple small clums, but they grow realllllly slow.
Anyways, mine seems to do the same thing your's does. It just jumps to bigger sizes by like an inch or more. My newest one jumped from about 8 to 10 in. in two leaves(the last one didn't work because of the cold weather we had in Florida when it was foming, so the pitcher died.). The new pitcher hasn't even opened yet so it might get a bit bigger yet.