This WIki uses the term LargeLowpans for Lowpans that are intended to scale to 10000 nodes and beyond.
It is not quite clear that Lowpans of this size can be built to the same general architecture as MidsizeLowpans. Nodes optimized for MidsizeLowpans may be overwhelmed by the amount of information needed to route in LargeLowpans.
It may also be hard to fit the administrative traffic (routing, service discovery etc.) for LargeLowpans into the limited data rates available.