How much are your blocks off? I have successfully blocked blocks that a as much as 1/4" off. Are all your blocks coming out too small or just some of them? I think if you cut all your sub-units larger you may run into other issues. I know this quilt uses different size blocks. I am thinking you need to pinpoint the reason why this is happening which can be anything from an inaccurate 1/4" S/A (or not using a "scant" 1/4) to too thick of a thread which can cause a block to be off if you have a lot of seams in it.