Comment 36 for bug 1672297

It's important to remember that RSS increases is no indication of a leak - it will rise and fall according to how busy the machine is and not how much new memory the process has allocated. Please ignore all RSS values as they are going to be unpredictable and misleading.

As for VSZ, that's kind of related to potential leaks. The problem with VSZ is that it's a measure of the address space mappings size and is also not a measure of regular memory allocated by the process. Although you can use changes in VSZ to detect leaks and bloat, address space used by VSZ is not an indication of the memory requirements of the process.

Probably a better field to monitor for actual leaks is VmData which you can find in:
  /proc/PID/status

VmData is a more realistic indication of memory allocated by the process.