The strangest thing is that, in theory, all creatures spawned in the arena are average in size/stats. I don't think they vary. Still, some will be bugged, some won't. Totally random.
They can vary. I left 10 GSW to air-drown, and butchered the corpses.
Bone: 1339 - 1690
Meat: 5596 - 7048
Intestines: 1382 - 1740
Fat: 1209 - 1523
Those two extremes came off the same two animals.
Really? I thought they were same-sized because the status screen always says "Average-sized".
Still doesn't explain why not all GSWs get bugged, unfortunately.
Is there any way something like DFHack or some other external program figure out the finer details of these creatures?
I think that what we've stumbled upon here is either a buggy feature or just a straight up bug, I believe that what it could potentially be is this:
Average size just means it averages around the given weight, so when a creature is spawned in it has a percentage of it's weight given or taken (e.g. 5%), or maybe a multiplier is applied (so that way there is natural variety in the starting creatures in a world). If it this is the case it'd explain why some GSW are buggy and other aren't, so some of them just about go over the weight overflow. However, what changes things is if weight and butchering returns are linked, because while the different returns given are fairly similar from what Sutremaine's stats show (apart from the meat, which is vastly different in comparison to the 1500-ish difference in the meat returns) then none of the whales could have been an overflowing weight, otherwise you'd get really small returns.
As an aside, if this percentage/multiplier is applied (and actually exists) to the GSW, maybe it does it twice, one for the SW and then, because they both have a size tag, it does it again for the GSW.
If DFHack (or something) can read into the weights more and figure out if these creatures really do have identical values then we can safely say that it's a bug. Also, if the meat returns aren't intentionally randomized, and not connected to weight (which is a bit easier to prove if we can determine that these creatures are the same weight) then it's very likely that while trying to clarify 1 bug we've discovered another one
(I hope I made any sense here, I'm not 100% sure that I did :S)