The amount of RAM at your disposal limits the number of copies of
Apache (as httpd or httpsd)
that you can run, and that limits the number of simultaneous clients
you can serve. You can reduce the size of some of the
httpd instances by having a cutdown version for
images, PDF files, or text while running a big version for scripts.
What normally makes the difference in size is the necessity to load a
scripting language such as Perl or PHP into httpd.
Because these provide persistent storage of modules and variables
between requests, they tend to consume far more RAM than servers that
only serve static pages and images. The normal answer is to run two
copies of Apache, one for the static stuff and one for the scripts.
Each copy has to bind to a different IP and port combination, of
course, and usually the number of instances of the dynamic one has to
be limited to avoid thrashing.