winIDEA stores various temporary files in %TEMP%/iSYSTEM, or if specified, %ISYSTEM_TEMP% directory. Most of the created files are meant to speed up your debugging experience and they remain there even after winIDEA is closed.
winIDEA will clean up such files regularly, but depending on winIDEA's workload, the size of this folder can grow considerably. If the system is running out of disk space, it is safe to erase the content of this directory.
Erase content of :
See also the winIDEA installation guide.