Muv's 100% correct! Shorter stitches put more thread in the seam....more thread won't "gather" the fabric as much as a shorter length of thread.
It also helps to wind a bobbin slowly...especially with poly thread. Winding it at full speed can stretch synthetic thread...then it relaxes (shortens!) as it's sewn...& puckers the fabric. Cotton threads don't stretch as much but it's still a good idea not to wind your bobbin at full speed.