Later firmware versions (specifically 6.2.x) introduced aggressive memory conservation techniques. The 200D is notorious for memory running high when using Web Filtering and IPS simultaneously. "Extra quality" firmwares include fine-tuned memory guards that prevent the device from entering conserve mode.
FortiGate 200D has reached its final development phase, with the last officially supported firmware version being FortiOS 6.0.18 fortigate 200d latest firmware extra quality
As hardware ages, turn off unused features. If you don't use IPS, disable it to save memory for antivirus or web filtering. Hardware Maintenance: The Go to product viewer dialog for this item. Later firmware versions (specifically 6