RSS (Resident Set Size) is a closer approximation of actual physical memory consumed by a process. VSZ includes shared libraries and anything else required to be mapped in to make the process complete but shared libraries are not in physical memory more than once for all processes except the writeable data area they may contain.
Presuming Linux or UNIX like system and CPython memory allocation ultimately comes down to the library function malloc which is layered over the system call sbreak. The malloc library call manages allocation of memory to whatever is requesting it such as the CPython interpreter but works from a pool for performance efficiency. If it runs out of room it calls the system using sbreak to get another chunk. It is possible to reduce the size by calling sbreak with a smaller size but malloc must be sure the area given back is completely free from use by objects. This was typically never done in the past. If malloc frees a lot of space then it will just reuse it on the next allocation request. I admit I have not dug into the CPython use of malloc reading code to see what it does. What I am saying is the numbers reported by system utilities can be misleading specifically if sbreak size reduction is not exercised.
