As the e-Commerce manager of this site, I've had the pleasure of working with Roland (the founder of REload Bags and the person shown in the pic) when we were both couriers working for the same company in 2001.
Not only is he a great person and makes great bags, he was never without his mini-Krypto U-lock. After noticing how almost every courier in Philly used one, it wasn't hard to know which lock made sure your bike would be there when you came back from a drop.
The KRYPTONITE EVOLUTION MINI U-Lock is small enough to keep in your back pocket, but tough enough to ward off pry bars, saws and whatever most thieves had on them.
I've still got my original lock from 7 years ago! If you need something small and secure, look no further.