I think you hit it on the head when you said that there are too many variables.

I once owned the Fender Stu Hamm Urge I that had a medium scale and body through strung bridge. It would use light gauge 34" strings, with some of the thick body of the string on the post. While this is not a wise thing, I never had trouble with the long scale strings breaking. The body-through feature probably kept the strings from going around the post when it would have on a normal bridge thru model. Since it didn't go much further than merely touching the post, that probably explained why I never had it break, even with long scale light gauge strings.
As for why there is a difference in the tension between the basses? There would probably be a lot of discussion and cussing on that one here. There seems to be a great difference of opinion on whether going through the bridge or having a shorter scale makes a whole lot of difference. I guess that would come down to the bass, the type and gauge of the string, and personal experience/opnion.
As for mine, I have noticed that shorter scales (at least the ones I own) tend to have less tension with the same gauge of strings than their longer scale brothers with the same make and gauge of string.
Then perhaps I am daft.....
