From everything I understand, a process's VSZ (virtual memory size) should be greater than its RSS (resident set size). Yet I see cases on a regular basis where this is not true. I am running Redhat Linux Server release 6.3.

Here is an example, looking at the process "Xorg":
$ ps -p 17316 -o pid,comm,rss,vsz,sz
PID COMMAND RSS VSZ SZ
17316 Xorg 314076 166932 41733

(I verified the RSS and VSZ numbers match what is in the /proc/PID/status file, to make sure truncation of the ps output could not be blamed.)
This shows that the RSS is almost twice as big as the VSZ. This runs counter to my understanding of RSS & VSZ.

Is there some sort of shared data that is calcuated in the RSS that is not also included in the VSZ calculation?
thx in advance...

The two are unrelated. VSZ is the total virtual address space which has been mapped by a process. Note that this can include unallocated space and space which is not resident. RSS is amount of space that is resident in memory and available to the process but may not be mapped. Either of the two may include space that is shared by other processes.

Thank you, smallpond. In reading your answer, I am assuming that there is some kind of memory that RSS accounts for - the part that you describe as "but may not be mapped" by a process, which is NOT included in the VSZ? Why is this? Is it because of extra space leftover in allocated pages in physical memory due to partial pages being requested/mapped? In my Xorg example I referenced, there were approximately 147+ mb more (almost 2x) in RSS than in VSZ. Perhaps this 147+ mb is the sum of the "leftovers" ?