I've heard they shrink too, or at least that's what the tournament guys claimed and justified 100mph back to weigh in.
Personally, I wreck 9" crappie when I try to filet them. The bones are just too soft for my Bubba. But I've had those days where the 10's are nowhere to be found and a keeper is a keeper to me. MDC does a fine job worrying about the heard. But I don't keep anything that is less than clearly over the 9" line.
I do feel like smaller fish fit farther through the mouth hole on the jig making them appear a bit shorter, so if my jig read's 9 I trust it.