There are these pesky things called the laws of physics.
The way around the need for a large resonance chamber and long strings to produce low pitch bass tones is to use a different technology.
If, for example, one were to stop relying on the large resonant box to produce sufficient sound, instead using electronic amplification, then it would be possible to reduce the size of the resonant chanber, even eliminate it and just have a solid slab of wood or other suitable material. Similarly, it would no longer be necessary to have 40-44" strings; something on the order of 32" or 34" would work. Now shortening the scale that much, hitting exact pitches might become more challenging, but it seems quite possible to use frets, as on guitars or the old viol family. Such an instrument would be much smaller and lighter than the double-bass. True, you'd need an electronic amplifier and speaker to make it audible since you've eliminated the resonant cavity - but that would be a separate device, and probably could be a fairly small cube or rectangle, say 18" on a side.
I haven't thought this thing out completely, but it certainly seems technologically feasible.
Oh wait, someone just showed me a picture of something like that...
never mind...