Paul, here are the dimensions as sent to me by Peter at PHD (I haven't spoken with him, but he has been quite helpful in email exchanges). The first figures as sent are half-circumferences in cm; next to them I've doubled and converted to inches:
Chest 80cm 63
Waist 76cm 59.84
Knee 57cm 44.88
Foot 49cm 38.58
For Ultralite, shoulder/hip/foot is 59/51/38
For Versalite, shoulder/hip/foot is 62/53/39
as reported on the WM website.
So some differences, but not great (except at the waist/hip?), and part of these could be how and where the measurements are taken.
Laying my Hispar 400/500 over a Versalite, there's not a huge difference in exterior profile that I can see.
But Brad, I think your implicit point is very good--the more interior air to heat up, the less efficient a bag becomes, depending also on how well its collar seals--a bad seal means air is constantly being expelled and new air has to be heated
Also, it seems that, down quality being equal, what matters is the average density of down per square inch of bag radiating surface--so a larger bag should be less warm than a smaller one, if each has the same amount of down.
As I mentioned at the beginning of the thread, my bag is a bit of a mystery. It was sold on eBay as a Hispar 500, but was some ounces lighter than spec, and the laundry tag inside had the "500" lined through. A warrant office (British military) number had been affixed. Peter's unable to trace the bag's history--military secrets, etc. As far as I can tell, it's a 500 from which some down has been extracted (Peter mentioned that PHD will do this if a 400 is ordered and they have none in stock, but do have a 500 in stock.) Doing some arithmetic, my bag is somewhere between a 400 and a 425. Even so, it's quite lofty, and the down does feel quite springy. The general sense is that it's very well made.
Filling it back up to 500 spec, with postage both ways, would have been prohibitive--better to put the money to another bag, perhaps a 600, hence the thread.