I've taken steps to reduce as much as possible the memory footprint of my Apache mod_php processes by disabling Apache modules as well as Drupal modules.
When the server is under light load, I see memory usage between 40MB and 80MB. However, when the server is under heavy load and there are about 100 processes active, 90 of those 100 will be 12MB.
I'm assuming this is a good thing of course. I know that the PHP is embedded into each process so then why would there be such a drastic difference in the memory footprints, 12MB and 80MB, and why can those first 10 be in the area of the 12MB range?