Memory footprint does not grow with RAM increase

This is on a QNAP TS251 NAS. It originally came with 512MB RAM and has since been upgraded to 8GB RAM in pursuit of better performance. The settings and process manager do indicate that 8GB is available.

However when I load the device with multiple large file copy etc... I observe that the CPU load increase but the memory usage remains at or below 512MB RAM.

My instinct says that something in the appliance limits RAM usage to 512MB overall despite more physical RAM. Is it possible in Linux? Where would you look to investigate? A Linux prompt is available.