Try glue basting the whole blocks together. This helps me find the problem when my block sews up too small. I found most of my problem is in the pressing of the seams. Connecting Threads cotton is 50 wt 3ply. That is too thick for top and bobbin for piecing. Try a thinner thread in the bobbin. I like Connecting Threads Pro on the cone. It's 70 wt and strong.