Looks like you have a lot of bias edges which certainly contribute to the stretching of some edges. Even a miniscule 1/32 of an inch carried over 8 blocks adds up to a 1/2 inch difference. Careful spray-basting will help, but what I'm currently doing to correct the same problem is to pick apart small sections of seam and push the edges either further apart or closer together and then re-sew. Love your design and your fabrics!