I am over 14 stone near 15 maybe and I have the 125, it pulls me no problem. When the flywheel mod was done to some bikes I have heard they can stall easily as Woody points out, I never bothered with the mod on my bike. Also I went down a tooth on the front sprocket as well. Only bug bear is you need to keep an eye on the carburation, it tends to flood or cough and stall easily on the 125 annoying if a steep drop in a section you need to blip the throttle, maybe not on the later 175s I don't know. As I have said before they are a cracking bike and low maintenance as well, none of that radiator nonsense to be messing with.
There is a 175 for sale in the classifieds on here, I have seen it and it is a tidy bike.