http://www.soccerstats.com/attendance.asp?league=england
Readings average attendance this season is 23,925. Their ground holds 24,200.
QPRs average attendance this season is 17,886. Their ground holds 18,360.
Both averages are only a few hundred short of full capacity and bearing in mind some seats are usually not used for security, I think it's fair to say that both clubs could easily fill bigger grounds.
Southampton s average is 30,520. That's over 2,000 short of their 32,689 maximum capacity.
Southampton are probably the bigger club of the 3 but I don't think anyone is disputing that. Also, as you quite rightly said, you have very little competition for fans in your vicinity, whereas QPR are competing for fans with 5 premier league clubs and 4 championship clubs within a 10 mile radius of themselves.
I don't know what has happened to Southampton fans. I always liked them as a club, but there are more and more of them claiming they will be in Europe and winning trophies within a few years. I don't know where all the optimism is coming from because as far as I can see, if they lose their next 2 games (Liverpool and Chelsea) they will almost certainly be in the drop zone and will be bottom of the premier league form guide with a non English speaking manager who may well be out of his depth.