Bought the extra small size.  Suspect it is the same details except the size of bag. **addendum:  All the pockets are proportionate to the bag size.  Just received medium.  Pockets are more numerous and larger than XS.  Much better for my big hands.  Medium is big but not as big as expected.  For my 6'2" it is ideal.  Shop carefully as you increase size price does not increase proportionately when on sale.

Extra Small size fits my note book if I turn it sideways or put it in the main compartment.  Basically this size is like a large purse meaning you do not have room for cloths.  Hung it on the car seat backrest next to me.  Handy to grab wallet, keys, pens, writing pad, water bottle, calculator, and other small things.  When someone else gets in I swing the bag around facing rear seats.  Well made in every way.

Liked XS enough to order a Medium size to carry cloths.  If I wear shorts and T carry long pants and long sleeve plus wind breaker "in case" weather changes which happens often.  These bags well suited to bike or bus travel.  People with cars can carry TOO much with ease.  Nice for travel or fairs too?  When you have too much to leave it all at home take some with you, LOL.

Up to Medium there are good discounts.  Large and above seem to stay closer to retail.  Medium size is good for a few books and extra cloths.  IHMO above medium good for more cloths and other LIGHT BULK.  Large and above will cost more per unit space but medium will hold all the books, heavy stuff, carried comfortably.  Add a laptop or notebook the weight will be serious weight on your shoulder.  Medium makes most sense for price/couple books/warm or cold cloths change/AND lots of small stuff in many pockets.

Of course you will pick the size that you think is best consider this;  XS is about 580cu", small is about 730cu"(+25%), medium is about 1590cu"(+220%).  Note the last MEDIUM doubles space.  It costs me maybe $5 more than small.  Next size Large jumps much more (50%?).  My cu" (cubic inch) estimates are off because the bags taper bottom to top.  However the top flap allows lots of expansion up and out.  My size estimates are just for comparison purposes based on listing.  MEDIUM is more for the money, if it suits you, than any other unless you find a special deal which come and go fast and in colors you may not like.

Suggestion:  prices seem to vary quite a bit from day to day or week to week.  Pick you color and size and follow for a couple weeks.  Saved me about 40%.  It would help to SEE and Touch various sizes before selecting.