Hi,
I had the same pb, i find that there was 4MB BrowserMetrics-*.pma files generated every 10 or 15s in this folder:
/volume1/@appstore/SurveillanceStation/local_display/.config/chromium-local-display/
Like me you're in a virtualized environnement with no GPU pass-thru so this is maybe why it is happening.. I don't know.
The solution is to STOP LOCAL DISPLAY service in Surveillance Station's application center.
If you want to get free space back, login your DSM via SSH as admin account and:
sudo rm /volume1/\@appstore/SurveillanceStation/local_display/.config/chromium-local-display/*.pma