I agree with you that a lightly built back can vibrate more and can have its own sound, but I differ from you in that I don't think this is a good thing. Every string pluck only has so much energy. If some of that energy, that could be vibrating the soundboard, is vibrating the back then the back becomes more a part of the voice. That sounds like a good thing until you consider belly muting. I love the sound of koalohas but I don't like the deadening that happens when I stand and play them because the sound gets muted on my body. I'm not trashing koalohas. On the contrary, I think they are fabulous Ukes. I just don't like the dampening effect.
I think all backs vibrate. Just because you make it thicker, doesn't mean it's not vibrating, just that you don't feel it as much.
There was an interesting TEDx with a luthier that has changed the way I think about this.
His view, is one that when I think about it... I have to agree.
When we play, we apply some amount of force to a string. That's a fixed input force, that causes a string to vibrate with that force.
Anything and everything in a uke, takes away from that force and uses it somehow/where else. If that use isn't to make sound, then the instrument wastes the energy. Some waste is converted to color when the wood makes a sound of its own, which can be a good thing, like a warmth. But other things just eat the energy and kill the sound.
Assuming you have 2 identically built instruments, with thicker/thinner back/sides, the force leaving the sound board, must be absorbed by the body of the instrument. There's two things the body can do with the vibration. Make sound, or not make sound.
When we hold it against our body and it damps, some of that vibration goes into our body. When you have a lighter build, where the body resonates, some of that energy is making sound. When you have a heavier build, the body doesn't covert it to sound, and just eats it.
On the one hand, a heavier built uke seems to damp less against the belly. But really a heavier built uke is just all around self damping. It's why they are more quiet. A lighter uke, is less self damping, but we notice the belly damping more because its already starting from a higher energy resonating level.