I've noticed that different fabrics sometimes require minor adjustments to achieve the correct pieced size. I found that batik fabric blocks were a bit smaller than those with regular cotton fabric, perhaps because the batiking makes it stiffer (?). I'd recommend whenever you are using unfamiliar fabrics or combinations of fabric, you make a test piece (not necessary to do same block as your project -- maybe just a simple 9-patch to check size). Then you can make any needed adjustment to seam size or even to cut size. Doubt this is a recommended procedure but it seems to work for me.