What are the dimensions of the XS versus the small sling? I would like to know the width difference.

The width on our slings is the same no matter what size: 25 inches.  So folded in half, the pocket would be 12.5" deep.  The length is what determines the size, and the size small is 4" longer than the XS.   A little insider tip:  the XS is truly very, very small.  If people tell you that you're the tiniest person they've ever met, then it's probably the right size for you.  If you're averagely small, then go for a good old fashion size small.