VIRT refer to the virtual size of a process - which is the total amount of memory it is using. For example this could be files on the disk that have been mapped to it, shared libariries and other memory shared with other processes. Hence it is not reliable when attempting to work out the total amount of memory used by multiple processes - since some of it can be shared amoungst them.
RES (Resident Memory) reperesents a more accurate picture of what the process is actually using in terms of physical RAM - it does not include any shared memory - such as shared libraries. And as a result is typically always smaller that the VIRT size since most programs will rely on libraries such as libc.
SHR reperesents how much memory other shared memory and libraries reperesent.
We can get an overview and totals of what memory is mapped to a process with the 'pmap' utility - for example:
sudo pmap <pid>
or
sudo pmap <pid> | grep libc
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