Of course United have a much bigger fanbase, their only rival is Liverpool in terms of sheer numbers. The reasons for that are myriad, but very little of this is relevant.
Attendances and fanbase are to very different things, although generally, clubs with a large fanbase can command higher attendances - providing their stadium allows for it.
The actual revenue generated by the fanbase itself is a relatively small portion of a club's revenue. Matchday revenue it limited by capacity of the stadium, and depending on how each club wishes to account for merchandise and food sales, the typical 'spend' of those fans on matchdays.
Then there's the wider merchandising issue, which again isn't really such a huge earner. Of course, every penny counts, but it really is small fry in the grand scheme of things.
United (and other clubs with large fanbase) will always use this as an example of their 'global appeal' and use it in negotiations with sponsors and advertisers, and in for media / broadcast rights (where they are able to negotiate a deal for themselves). But that's only THEIR side of the negotiations. The sponsors will look to their champions league presence, their domestic success, they players they have, and the image they are currently portraying.
It would be foolish to just dismiss the big brand that United are. It still holds a fair bit of sway, but there are limits to how attractive that is to some sponsors. When the going is good on the pitch, sponsors will milk the 'heritage' and big name of United - because it's in their interests to do so. But when the going is not so good on the pitch, it becomes far less a valuable commodity. So what is it's a big brand... it's a losing brand. It will command less money, and will be deemed a tired and old hat brand.
City on the other hand (and really, lets not kid ourselves) don't have the huge global following of some of the big boys (yet). We're emerging. Of course we have fans all over the world. nobody's denying that, but we are still very much growing.
From a sponsors perspective, we are everything that many 'traditional' big names arent. We are new and exciting, 'going places' and changing our dynamic very quickly. That's a very positive thing to be associated with for most companies.
For instance, if you're 'Yorkshire Tea' or 'Sotheby's Auctioneers' - you might be looking to sponsor a cricket team, or perhaps a club with a more subtle image, but if you're Red Bull, or Apple, you might be far more interested in a team that reflects the same dynamic as your company (e.g. City).
The fan base of clubs really isn't such a major factor in attracting sponsors / partners and advertising. the image of the club and the reach of the the club (typically through media exposure) matters far more.
As things stand right now, United's exposure is greater than City's and our image is currently far more fresh and vibrant than ANY other club in the PL and probably most of Europe too. As long as we keep winning things, and don't get too many awful headlines, we'll be able to sell ourselves better than United can.
We also have more drive and impetus behind us because the owners have been planning this for a number of years, and are putting it all into practice as we speak. United on the other hand have been sat back and have only just come to the realisation there is work for them to do. That effectively puts them a number of years behind us in terms of 'readiness' to grow rapidly.
It would be arrogant (and foolish) to think a behemoth like United will just fall like a pack of cards. It won't. It will react. But it's slow to react because it's not agile in the way we are.
That's why (in my opinion) if we continue on the path we're heading, and there are no overwhelmingly difficult hurdles, we'll go on to surpass them in business capability and hopefully profitability. But there's a fair few ifs there. We still have a lot to do, and can't take our eye off the ball.