Thicker threads can make you lose some size as well as how you press your seams.
Also you may want to try sewing a scant 1/4" seam. It is approx 2 fabric threads less than a 1/4". If you can adjust your needle position to the right, you might try this on some scrap practice blocks and see how it works for you.
If you can't, you can buy some moleskin and lay a strip of it a scant quarter in to the right of your needle and use it as a guide to run your fabric next to.