In a room with 5000 people, it's a lot more convenient to use a DS to communicate with people, as where in a room with 100 or so people, it's almost inconvenient to use the DS. Also, with so many people concentrated in the same place at a larger con, the chance that many people not only have a DS but are using it is increased significantly as where at a smaller con you're simply not going to find as many people using them. Perhaps the ratios are about the same, but the sheer numbers are still fairly minuscule.