Am I correct that the Yaris has an inredibly stiff chassis?

[Loren]

Yes, it is quite a stiff chassis.

There's a spec for "torsional rigidity", something like weight/degrees of chassis flex or something like that. It's pretty darned small on most modern cars.

The increase in stiffness has more to do with the market's desire for/expectation of less NVH in a car. Toyota is trying to build a small car with the appeal of a bigger car. Those of us who grew up in the 70's-80's think of small cars as being cheap and rattly little things. Toyota (and others) is trying very hard to defeat that reputation.