Prism99

This is one reason I don't make entire quilts out of HSTs; mine are always like that! Honestly, I think the reason is because you are sewing along a bias on both fabrics. My reasoning is that the feed dogs move the bottom fabric at one pace while the presser foot is stretching the bias of the top piece.

One way to test this theory is to cut 2 pieces of paper 4.5", mark them, as sew just as if it were fabric. Paper does not have a bias so theoretically the result should be more perfect than with fabric.

If the paper turns out better, then it would help to *heavily* starch the fabric before cutting. Although this won't completely stabilize the fabric bias, it will reduce any stretching or distortion that occurs while sewing.

Using a walking foot would probably help too, especially if the fabric is heavily starched. Reducing presser foot pressure might help too.

I think some sewing machines cause more distortion than others because there is more of a discrepancy between the presser foot and feed dogs.

Edit: Or, my theory could be completely wrong and the discrepancy comes from slight variations in cutting, sewing, and pressing.

For example, especially if using steam, ironing from the right side, and ironing the seam to the black fabric, the iron could be stretching the black fabric out of shape. Some fabrics have more stretch along the bias than others, and if you are moving the iron from the printed fabric to the black fabric along the seam line, you are pressing the black fabric along its bias. Actually, this may be the better theory because the discrepancy is bigger at the corner of the black fabric, which is consistent with the iron distorting the fabric.

Are you pressing with steam? Pressing with a dry iron and using only the tip of the iron to press the seam might help. It would be moving the iron across the black fabric that would distort it. The bias runs not only along the seamline, but also perpendicular to that line, ending at the corner of the black fabric. The printed fabric, if on the bottom while ironing, would not be as distorted because you would be ironing it towards the seam line rather than towards a corner.

Sorry, probably too much speculation here......