Those bias edges from the 1/2 squares stretched.Just top stitch 1/8" away from edge using a fairly long stitch length ( 3.5 will do) . Then use a pin, lift bobbin thread and to pull a bit of fullness out of each triangle. A bit of work but saves grief. When there are much smaller triangles, jut tighten the machine's upper tension ( quite a lot, like a whole #), and stitch that 1/8" edge stitching. Works for me!