I have small arms too and find the Belkin armband quite good. I bought one from
Amazon a couple of years ago. Bought another one recently and thought I'd try a different brand so opted for the Tunes one also from amazon. I don't find the fit as tight as the Belkin and the phone doesn't fit so well either.
Let me know if you find a good one! Not sure what the Adidas or karrimor ones are like.
Edited: 09/12/2012 at 17:45