If you use top in Terminal, you may occasionally see apps with huge VSIZE values. I know this because Witch, one of our apps, is an example of such—it's VSIZE can exceed 11GB.
In trying to figure out why this was so (short answer seems to be: we can't control it, but it's not a problem), I ran across an interesting command, vmmap. This command will spew out a ton of detail about virtual memory usage. Stringing a couple Unix commands together, though, you can extract just the summary portion of the report.
In Terminal, run this command:
vmmap -resident PID# | grep "Summary for" -A 999
Replace PID# with the process ID you're interested in observing, and you'll get a nice summary of its virtual memory state. You can see what's virtual, and what's actually resident. Here's a bit of the output for Witch, for example:
...
REGION TYPE VIRTUAL RESIDENT
=========== ======= ========
...
CG shared images 1280K 1256K
CoreGraphics 16K 8K
CoreServices 6232K 6232K
IOKit (reserved) 256.0M 0K
MALLOC 337.3M 7932K
MALLOC (reserved) 7.8G 0K
MALLOC freed, no zone 72.0M 212K
MALLOC guard page 104K 0K
...
As you can see, although Witch has an 11GB virtual size, the biggest chunk (7.8GB) isn't using an real RAM—and best as I can tell, will never use any real RAM.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20121016063646746