You guys get the "American" spec suspension... we've got the "Euro" spec, so I really can't relate. The Fiesta drives better than a Mazda2 or a Honda Fit (here), with the main difference being that the Fiesta's steering feels like garbage in comparison.
Your Fiesta is apparently so wallowy even the old Honda Fit feels better. Which is downright strange.
Weight comparisons between the Sonic and Fiesta don't mean much, because the Fiesta has a tiny back seat. The Fit, on the other hand, is exceptionally light for its size.