I've built my own longbike, and an actual xtracycle for my wife. We're beginning to think that they're longer than we need for 90% of the time and considering something a little shorter. I'm not sure it really makes any sense to go shorter, because the length isn't much of a problem, but i just like messing with stuff!
Anwyay, i'd noticed that the Kona Ute and Trek Transport appear to be significantly shorter than the Big Dummy or Xtracycles, just judging by the apparent distance from the rear wheel to the seat post.
I decided to collect as much info as i could find from the manufacturers websites and put it in a table for comparison. The relevent info is primarily wheelbase and chain stay length. And i added a row for a computed value that is how much longer the chain stay is than that of a supposedly average bike, with a 44cm chainstay.
Below is the info for your reference. It's interesting to note that indeed the Big Dummy and the Sun Atlas (and i suspect the Yuba Mundo as well, though i couldn't find numbers for it, however it's wheelbase is even longer than the Sun) are indeed about 20 cm longer than the Ute and Transport, and even a couple cm longer than an Xtracycle.
(Updated to correct for 700c wheels on Kona, update CS lengths, add home made bikes)