This purse is knit at a tighter than normal gauge for the recommended yarn weight to help keep its shape.
From Knitty.com:
This is an excellent everyday purse with two pockets (one under the flap, and one that nicely divides the interior), with enough room for your wallet and lipstick, maybe some keys and your i-pod and a note pad to write down good ideas when they come to you.
It’s made in four pieces from sturdy mercerized cotton. The front flap and strap edges are finished with a knitted-on i-cord that provides stability and gives a nice neat look.*

Note: Accurate row gauge is important in this pattern.