I use the small ones because Alex Anderson from Simply Quilts recommended them. I think the reason is that she thinks there is less of a chance to have shifting and puckering on the back during FMQ. Per her suggestions, I put LOTS in my quilt; about a hand width apart all over my quilt and I have never had a pucker in a backing. I never thought about the weight issue that Mucky mentioned by using the larger pins. I bet my quilts would be quite a bit heavier since I pin so densely.