You will probably notice that the X cars have an exceptionally strong, solid body, even compared to other GM cars (which are notably of such construction). The similar FWD A bodies aren't quite so rigid. This makes these cars hold up well in accidents, and to protect their passengers well also. This is because of galvanized steel construction used in the body panels.
Also worthy of note is the use of a subframe or K frame, underneath the engine and transmission. This isolates driveline noise as well as reduces rattles and squeaks as the mileage builds up.