I think we were always led to believe that support was pretty even in the city, but that the oinkers had a wider appeal outside the city than we did, possibly due to their unusual name* and ground being used for FA Cup semi finals.
*I don't think it ever really meant much in terms of feet on the terracing, but people do latch onto teams with memorable names (being named after a day of the week...) like they did when 'Young Boys of Bern' came to prominence from the Swiss League, as an example.