There are lots of parts to this puzzle. The traditional definition of tension is pretty straightforward, and is a function of string, pitch, and scale length. Longer scale=higher tension at the same pitch & the same strings. BUT... as mentioned above, other things enter into it when you're talking about the practical aspects of guitar playing - mostly that means bending.
The same strat will feel different if lock nuts are tightened down - that's because even though the tension is the same, there is more string length to stretch *without* the lock nut. It'll feel "looser". That's why non-locking reverse headstocks feel different - they're looser on bass strings, tighter on treble strings than "normal". Again, this is only while bending - the overall tension is the same.
With the jazzbox tailpiece, you gain a lot of extra string length to stretch, so bends are easier. Of course, with typical jazz strings, it still ain't that easy... ;-)