I played around with a Bessa III, not really sturdy and not really small, even folded. However, I'm sure I could carry one all day in my coat pocket or even on my shoulder.
A positive vote from me for the Fuji GA645 series, pretty freakin portable for a wide MF, fits nice in your hand, lightweight and really robust.
I would buy the GS645 folder, but it's kinda complicated because of the bellows that need to be replaced. So I'm saving up for the Bessa III...