Yes, it would have been a lot more, but there was political pressure to make it seem much smaller and so easier to pin on a few larger louts (which were the social bogeymen of the day).
For this march, even if the police were bussing protesters in (and I really wouldn't put it past them) 100,000 isn't really that much. There were more people who couldn't get Oasis tickets for Manchester, for scale.