Tried on the Reg Small in the store, and while it fit, it was slightly roomy. This isn't uncommon for me, I'm 5'1 and on the slimmer side. I have never bought Old Navy's petite line. But in the absence of a Reg XS in store and online, I discovered the petite line. In my experience "Petite" usually... is a reduction in overall garmet proportions to fit better on little smaller builds. Having good experience in other brands' petite line. I figured I'll try them out. The Petite Small fits BIGGER then the Regular Small, and is...significantly shorter. The dress is a short design. But the Petite Small is slipping off my shoulders, and only just about covers my behind?? While I expected differences in fit, these issues were not REMOTELY found in the regular small. The petite somehow is LARGER and WAY SHORTER then the reg. This is NOT petite sizing.