The bias edges got stretched, and that is what makes them "D-Cups".
You have to be VERY careful not not stretch the edges, just let the feed-dogs pull them through. You have to be careful when you cut, plus you have to be very consistent about your 1/4 inch seams. I always sew from the outer edge toward the inner tip. I don't know if that helps, but I've never had anything but flat hexs, and I've done a bunch of OBW's.
You can tell if your seams are correct when you get 3 triangles sewn (a half-hex). If the edge is straight, you've sewn correctly. If it bows, your seams are not consistent.
Keep practicing and you'll get it. Good luck and let us see the finished product.