Moisture makes cotton fabric shrink. Starch contains moisture and can therefore make cotton fabric shrink. This can be used to our advantage, as shown in the following Sharon Schamber videos:
Sharon Schamber's Beginning Quilter : Magical Basting P1
http://www.youtube.com/watch?v=pi1RgiLI9GI
Sharon Schamber's Beginning Quilter : Magical Basting P2
http://www.youtube.com/watch?v=PmcdOz-ypwU