I would quilt it in sections. You can cut the backing the same size as the top, but cut the batting into 3rds. Sandwich the quilt with the first third of the batting in the center and pin as usual. Quilt to about 1" of the edge of the batting. Now take the next piece of batting & butt it up against the first. Whip stitch the 2 pieces together. You can also use a really big zigzag. Sandwich this portion, pin & quilt. Repeat for the other side. This way you really reduce the bulk, just take care you don't catch the unsandwiched portion of the top or backing!