I think people tend to quilt more heavily with long arms simply because they can. There aren't the practical barriers of time and physical effort that you have when stitching by hand or when trying to move a heavy quilt under a needle.
I think what people are referring to when they say that a lightly quilted quilt is more snuggly, is that the batting retains more of the air. For any particular stitch in hand quilting, you have the thread either on the top of the quilt or on the bottom, but not both. With machine quilting, a top and bottom thread meet in the middle with every stitch and compress the batting.
I try to keep that in mind when considering a design for a machine-quilted quilt. I figure that I should have about half the amount of stitching when machine quilting that I would have when hand quilting if I want a similar effect, since I am using twice as much thread and compressing the batting with each stitch. I think this is why heavily quilted machine-stitched quilts seem stiff. The air has been quilted out.