So I did a few quick calculations on the raw numbers - like the average length was just under 46' LOD. And every kind of boat you might imagine has made it 'round. But there was a surprising number of Open 60s, Volvo boats, and other race craft. Plus several boats longer than 100' LOD. Nothing against megayachts, but it seems commercial craft like the Picton Castle, or the record-breaking G-Cat Team Adventure should probably be excluded, as well as the boats which were clearly engaged in a round-the-globe race. That brought the average LOD down only to about 44', still longer than I had expected.
I guess I was expecting the average would be closer to 38-40. It seemed to me there were an awful lot of gold-platers in the list, and a dearth of Vegas and Contessas and their ilk. Even a couple of boats I personally know of were missing. Maybe the amount of money thrown into such a project determines how likely you are to be successful, or at least to be reported to the magazine.
On the internet, nobody knows you're a dog anchored in a coral atoll.
I would have thought your original guess was on too. I suspect that those 44 foot average were suitably crewed and might not have reflected the mom & pop cruisers too well. But then what do I know about it. I've known circumnavigators but I'm not or ever plan to be one.
I would also think that if the list was primarily US based that the average size vessel would be larger. Americans seem to view 30/32 footers as being "small", perhaps because of the sheer numbers of boats, and their realtive cheapness. in the USA.