I've noticed in practice pieces that the size and weight does affect stitch size. A walking foot should go a long way to fixing that problem. Also providing support for the weight of the piece as you feed it into the machine will help. I've been experimenting with ways to support the quilt. Leah Day rigged up an interesting system from the ceiling for one of her recent projects.

Cheers, K

I think it is a walking foot problem. A general foot is not going to feed the fabric thru properly. Because you have older machines, you may have to do a little research to get a compatible walking foot, but I know they ARE available.