I noticed my site was allocating 2048m of memory to each PHP process, far greater than what I had specified in php.ini as my default and considerably greater than what I had set in this particular site's wp-config.php. Upon further investigation I found that the Sucuri plugin was setting
@ini_set('memory_limit', '2048M');, but not just for some cases, any time the plugin was enabled. Is this by design? As it stands I have it commented out as it was causing issues when the server is under load.