It probably is bulk. KwikSew patterns for knits always used 1/4 seams. Knits sewing seemed to start being popular after zigzag machines with stretch stitches appeared. Most knits don't ravel very well.
Quilting is also contained stitching - the seams stay in the back of the block and are not open to wear and tear.
5/8 gives you room to finish the seam with garment sewing. It allows for an French seam.
The one seam that bothers me is the 1/2". I've seen heirloom patterns use it.