If you are referring to the lettuce edge itself (not the gathering of fabric to make ruffles), then the amount of curling is really dependent on the ability of the edge of the fabric to stretch. The more the edge stretches while you are serging, the more curling you will get. If you are inserting fishing line within the stitching, then I would say the more fabric there is in the ruffle the more curling you will get with the fishing line, because there will be more fishing line to make half-circles.
Clear as mud?
Sorry, if I knew more about Irish dance dresses I might be able to respond more intelligently......