The virtual size of a program includes all shared objects (shared libraries, shared copy on write memory pages, the read only executable pages and other shared memory) that a process uses in addition to the memory that only that particular process is using (its resident set size.) Shared objects like shared libraries can be mapped into memory by multiple processes and thus don't use any additional physical memory for each additional process that uses it.

The RSS thus usually indicates the amount of heap and stack a process is using that is unique to it.