please explain how principle that using fortianalyzer makes memory decrease effectively

i have a question. i have a Fortigate-500D Because of high memory usage i'm considering introduction of fortianalyzer. engineers using fortigate of other company say "when making disk logging disable and disk format(unit reboot) then using fortianalyzer, then they could get about 20~25% memory decrease effectiveness." please explain how principle that using fortianalyzer makes memory decrease effectively