Time to switch to diet coke ;-)
Just kidding, I'm a fat bastard too. I've also got the CMI. I like it overall better than the petzl, but the strap isn't brilliant. The little toothed adjuster that holds the strap chews them to bits, and then eventually starts slipping. Did it break at the adjuster? Could be worth contacting whoever you bought it off, they might man up and give you a new one or at least put a new strap on the old one. You could probably make a new strap yourself, it isn't all that hard to do.
Shaun