are you stretching your blocks on purpose?
if not- try (pressing) not ironing! which means you place the iron straight down (press) it's not a shirt---you don't want to move the iron around on top of the block-
whether or not a block will go back to it's original shape/size after quilting probably depends a bit on the batting being used- the density of the quilting- how it is handled. if you want it to stay stretched you may want to use a batting that will not shrink (poly instead of cotton) and quilt it heavily-so the quilting holds it but it would really depend-every one will probably behave differently with lots of factors involved- stretching them on purpose will generally wind up with skewed blocks in the end.