In reply to Gemma: Why not just buy a small DMM Renegade and swap your current leg loops over? The waistbelts and leg loops are interchangeable on the Renegade - and all other DMM, WC, BD and Petzl harness I've looked at recently.
I might even have a spare Renegade still sitting around I'd be happy to sell, if you do want to try that.
Equally, contact DMM or WC direct and see if they are willing to sell you a harness with small waistbelt but medium legs. If all else fails, buy two harnesses and try to sell the other 'one' to someone with skinny legs and a fat belly [although I can't help on that as I'm skinny with even skinnier legs.]