To be honest I've never minded Liverpool, if only for the single reason that they stopped United from winning anything during the 80s. They have some misguided fans, but reading some of the above, so do we.
City were one of the biggest clubs in the 20s and 30s, but so were Huddersfield and Bolton. Liverpool were poor in the 50s, but recovered in the 60s. That's the way football evolves. Football is cyclical and after 30 years of misery it's our turn.
As for what defines a big club, I would guess that around 30 clubs from Spurs and Newcastle to Sheffield Wednesday and Birmingham are all capable of 50,000 crowds if they were successful. Similarly, if City or Liverpool dropped a few divisions, our support would soon diminish. I know we had decent crowds in the late 90s, but for all sorts of reasons, I'm not so sure that would be repeated if it happened again.
Bill Shankly was a genius and so was Joe Mercer. I was genuinely happy for them when they beat AC Milan in Istanbul and I know for a fact that the Liverpool fans at Swansea were going mental when Aguero scored against QPR.
The big danger is that we start acting like United fans. Let's enjoy our success without the nadtiness and aggression that follows the rags. Joe Mercer believed in football with a smile so why shouldn't we?