well i,m going to challenge that claim
My time in Birmingham which was about 5 years on and off I would say that the bulk north of the city was Aston Villa land whilst the south is Birmingham city . Obviously being a huge city there are fans of both sides in all areas
I agree with villa being the biggest and best supported but for some reason they attract alot of fans outside Birmingham whilst Birmingham don't apart from Redditch . Villa have a huge fan base in most border areas like south staffordshire .Lichfield and Tamworth for example is almost 100 per cent villa .If Birmingham were in the prem they would get big crowds too