For me, I've gotten shorter stitches as I practice. But, I also have noticed that my shorter stitches come w/ shorter needles(took a moment to get used to holding on to them). The statement above about my quilt being a bit looser in the frame as also helped and the type of batting. The thinner the batting, the smaller my stitches.
The other thing is going thru layers - If I'm quilting thru a seam allowance, my stitches will be longer because I'm going thru more layers...Just my observation.