The web page only uses memory when it generates the page and process
the request.
There is no web server continuously running.
The same happens for ftp/ssh/telnet,etc, all are handled by 'inetd',
the small super server that is always running and only launches the
needed server when a request arrives (and the launched server
terminates as soon as the request finish).
This way memory is conserved.
As scaramanga wrote, use "top" to see the (virtual) memory
consumption of each running process. If the %VSZ goes behind 100%,
it means that the process is bigger than the available physical
memory and swapping to disk is always happening while it runs.