I think Roanoke's sphere of influence is larger (in land area) because it is farther away from competition than Richmond or DC. If you look closely, a good 20% of the Roanoke sphere is in West Virginia, and it extends farther southwest, where competition from Johnson city is weak and Knoxville is the first large city. Richmond is really feeling the squeeze from Norfolk, DC
and Charlottesville. And notice that the Richmond sphere extends the farthest to the southwest, toward Roanoke. Poor DC is very close to Baltimore, severely reducing its dominance in that direction. Look how large Billings Montana's sphere is, despite its small population, because there's no competition. I think oftentimes people relate to the closest city that provides certain services, especially the city that is the hub for the media market for their area. I wonder how closely this commonCensus project would mirror the media market boundaries of the country?
Another factor is uneven participation. Richmond and Norfolk are probably under-represented. Look at Los Angeles, which has fewer participants than San Fransisco, despite being significantly larger.