Magento CACHE folder very big?

i have been checking in the magento folder of /var/ and i notice whenever i clean FPC in my magento admin the “cache” folder doesnt get cleaned the file size remains the same – how should i clean this ?

what is strange also there are 2 files “cache_fpc” & “cache_fpc–” when i clean FPC inside magento the file “cache_fpc” changes and gets smaller however file “cache_fpc–” remains the same size ?

what is file “cache_fpc–” ???

magento is 1.9.3.8 version

Regards