Disk size keeps shrinking
Ah, these are core dumps. You can find more information on why they happen and how to prevent them here. In a nutshell, they are generated for debugging, but shouldn't be kept on disk by default. Either the system setting is wrong, or some buggy process is choosing to save them. Running 

sudo launchctl limit core 0 unlimited


in Terminal will disable the system from saving them by default, but still allow processes to make them if they choose to, or running 

sudo launchctl limit core 0 0


will disable them entirely, both in the system by default, and by disallowing processes from creating them. There should be no issue if you disable them completely.