linux - vsize in proc/<pid>/stat

I would like to know the difference between vsize in stat and VSZ (using ps aux). I'm assuming these are the same but the values are different.

For PID 1:
    vsize in stat(/proc/1/stat)     :  654425776128 (Field 23)
    VSZ using ps aux                :  8944

I want to know what are the units and conversions b/n these two.