PS RSS VSZ

rss RSS resident set size, the non-swapped physical memory that a task has used (in kiloBytes). (alias rssize, rsz). 

vsz        VSZ      virtual memory size of the process in KiB (1024-byte units). Device mappings are currently excluded; this is subject 
                    to change. (alias vsize). 

size      SZ      approximate amount of swap space that would be require…

rss The actual overhead physical memory, which is the physical memory currently being used by the process.
vsz The virtual memory size, that is, the size of the memory that the program is not currently using but may be allocated.