Yes, London was big, and the two Wikimaniae since have been on a smaller scale, but I'm not sure a ~1,000-person conference is significantly less of a headache than a ~2,000-person conference, and actually I'd wager that Esino was more logistically complicated due to the location - for example having to arrange buses to Varenna and the airports (which were around 50 miles away). Not that that should be taken as a criticism of the Esino team - they did a fantastic job in a beautiful location and I'd love to have another 'scenic Wikimania'.
I'll let Ed tell you about what he did. I know I saw him spend a lot of time dealing with the venue and the programme and discussing finance and logistics, but I'm sure there are lots of other things. Speaking for myself: those volunteers in red shirts? That was my contribution. The volunteers on the helpdesks, running sessions, meeting and greeting, tweeting, photographing, doing odd jobs and generally making things run smoothly ... I recruited most of them*, got to know them, trained them, split them into teams, did a lot of the scheduling (easier said than done - lots of moving parts!). During the conference, they looked after the attendees, and I looked after them. And I've never worked with such an amazing group of people. It was a truly humbling experience, but it was a lot of work. At one point I was receiving something like 200 emails a day just relating to Wikimania and was having to set aside time at the start and end of the day to answer the ones that didn't require an immediate response. I also devised the scheme of reporting and emergency/contingency planning for volunteers (thankfully this wasn't necessary, but the death of a Wikimedian at that year's Wikimedia Conference was painfully fresh in our memories), and spent a lot of time trying to drum up and channel interest within the UK Wikimedian community. I'm sure there were other things, but those roles alone took up a significant amount of time - certainly in excess of 40 hours a week in the final few weeks before the conference.
*(Not wishing to take credit from anyone else; I worked closely with lots of other people on all these things, particularly Hera Hussain, and Fabian Tompsett and Chris McKenna who were at the time employed by Wikimedia UK.)