How process size is calculated? Is it always based on the current highest available address in memory space?

[Yuri]

malloc(3) can be controlled by MALLOC_OPTIONS to use mmap-based 
allocation as opposed to sbrk-based allocation.
But allocations/deallocations with mmaps can eventually lead to 
non-continuously mmapped memory (having some non-mmapped gaps).

Are  these gaps excluded from the process size or size is always linked 
to the current highest available address in memory space?

Yuri