I agree, some machines stretch things on the bias when they stitch....so test sew on the biases before starting a project with a lot of bias edges....I think sometimes adjusting the foot presser, the thread tension or stitch length can help with it. If you have a serger that has a differential feed adjustment knob, you can see that in action...you can just turn that knob to get gather or to get ruffled edges ...amazing...but alas...not so good on a sewing machine. I love my Viking D1 as it usually gives me great seams on biased edges and I think it has something to do with that "hover" function where my presser foot raises just a bit between stiches...allowing the fabric to relax a bit between stiches