I think the belief is that when you expand the area you introduce people into the system (either their home or work) which are new and likely different from the existing traffic patterns. As the diversity of traffic patterns increases, the ability for each individual bike to have multiple trips per day increases. i.e. if it was 100% commute driven, a bike would get 2 trips per day. NYC sees something north of 9 trips per bike per day at the peak so far. That's double the trips per bike per day of any of the other systems I've been able to find stats on, so NYC is already doing well.