I'm guessing ECU looks bigger because it has all of eastern NC to cover, while UNC & NC State are competing for essentially the same fans.
Notice there are very few DOOK fans on the map, though...
Total votes (as of the date of the last map):
East Carolina 238
UNC 199
Duke 164
NC State 112
Wake 41
Duke's fans seem concentrated around Durham...
Actual attendance this year:
NC State 56K+
UNC 48K+
ECU 37K+
Wake 32K+
Duke 19K+
I don't have TV/radio ratings readily available.