And a crowd of 15,000 is small according to what calculation? Maybe the O2 isn't the biggest venue in the UK (which I didn't even say - I said it was the biggest indoor venue I've ever seen), but 15-20,000 is pretty much the biggest category of indoor venues in Europe. Take Berlin, a major urban centre with every size of venue available. Maiden played the O2 World there last year, which is perhaps a tad smaller than the O2 in London. It fits an audience of 17,000 for concerts, and it was sold out (something I know for a fact). There are only two bigger venues in town, and both are outdoors, one which holds 22,000, and the Olympic Stadium, which holds something like 70,000. The latter hosts none but the biggest international acts, and it is hardly ever sold out. I'm talking about Robbie Williams, The Rolling Stones and AC/DC here. Maiden falls well short of that category, as much as we love them.
So, just because they aren't football stadiums doesn't mean the venues Maiden sell out aren't huge.