I'm 5'1" and weigh about 130.  I wear an 8P in jeans which depending on how they are cut either fit perfectly or are serviceably snug.  I've accepted that over the years I'm no longer the XS or S that I once was so I intended to order these in a medium.  After reading the reviews I took a chance and changed  to  Small.  I'm so glad I did.  They fit perfectly with just enough slack around the hips that I'm not worried about a bit of shrinkage.  Naturally they were too long but I'll wait for a month or so of washings before I make any permanent changes to the length.  I liked these enough that after a few hours of wearing them I ordered another pair in the dark blue.