your cable chain stitch and how heavy your buttonhole thread is.
on each loop, it’s helpful if you pull the first buttonhole stitch a little snug, and then work the middle buttonhole stitches on the loop a little looser, and then work the last buttonhole stitch on the loop a little snug again. this will help achieve a nice rounded shape to each loop.
don’t crowd the buttonhole stitches on the loops. if you crowd the stitches on too tightly, your buttonhole stitches will ruffle up.
you don’t need to end the buttonhole with each cable chain stitch. once one side of a cable chain is filled, just step down to the next 