Why is the maximum memory usage 512?

Why does WordPress core/plugins Memory/CPU usage vary so much?

• Putting the finishing touches on a WordPress 3.8.1 site based on Genesis 2, I keep wondering why memory usage has varied so wildly during development. Using the TPC Memory Usage (updated) plugin, I measured swings from ~40MB to almost 60MB RAM for WordPress, even after I migrated the entire site from SiteGround (with Varnish, Memcached and whatnot) to a plain-vanilla Plesk-based Apache server setup. From the moment I changed PHP to use version 5.5x on the Plesk/Apache server, WPs memory usage has not gone over 12MB RAM, with the same content, same theme, same plugins, and with the client adding and editing content. I don't quite understand why memory usage would change so much, given that the basics besides the PHP version have not changed. Also, if this has to do with emptying some RAM-based content or cache or whatever, is there a way to force that without switching PHP versions?

• Answer:

  I must say, 12MB is pretty low! But from what I read, the memory consumption has dropped considerably from PHP 5.3 to php 5.5. If you also leverage OPCache, that might be the reason why your memory consumption is so low. Not totally unrelated; I recently discovered that translation within WordPress also takes quite an amount of memory.