If you have a large quilt and a small harp on your machine, one way to get around this is to separate the batting into 3 strips. You machine quilt the center part of the quilt first. The side rolls up easily to fit under the arm because there is no batting in there. Once middle is quilted, you attach one of the side battings and quilt that.
Debra Wagner explained this technique in detail in one of her books. Now Marti Michell (sp?) has a book out on how to do it.