Mark,
You're right that as vehicle weight decreases then the max torque spec generally goes up, but it's an indirect relationship at best.
The engineering reasoning behind this is based upon the notion that it is the duration of time at peak load that varies as vehicle weight increases/decreases. Weight doesn't actually affect the peak loads experienced by the transaxle - the engine is putting the same peak forces through the transaxle regardless of weight. The difference is that a vehicle which is heavier generally exposes the transaxle to longer periods of peak loading, and this in turn affects the long term durability and reliability of the transaxle. Gear teeth and bearing surfaces, for example, wear at a rate dependent upon both force and time under load (among other factors) - heavier vehicle correlates highly with more time under load.
So to relate this to the larger discussion, it's somewhat of a mistake to equate lighter weight with a higher permissible instantaneous peak load rating. Said another way, just because the car may be very light weight the little ole' Audi box can't handle a 650hp/tq big block burn out.....unfortunately. However, most of these Audi boxes aren't that expensive compared to a ZF, so no big deal if one implodes. I keep extras under my work bench for that very reason!!