Icons created at runtime are now purged from the .dyn.rsc cache file following some reboots and when the world starts up. This prevents the dynamic cache from growing extra-large from icons it has no need to keep. Icons and other files uploaded by users are still kept, according to the setting of world.cache_lifespan.
This issue has been resolved.
Icons created at runtime should be cleared from .dyn.rsc between sessions since there is no way to use them after a session ends.