FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.

Enabling "brief-traffic-format"  in " config log setting" reduces log volume by omitting some log fields.

config log setting

set brief-traffic-format enable
If you enable the above setting, the following log fields will not be available:
srcname, srcuuid, dstuuid, poluuid, user, groupappid,  appcat, apprisk, app (except local traffic), vpn, vpntype, shapersentname, shaperdropsentbyte, shaperrcvdname, shaperdroprcvdbyte, shaperperipname, shaperperipdropbyte, devtype, osname, osversion, unauthuser, unauthusersource, collectedemail, mastersrcmac, srcmac, msg 

Be aware that enabling this feature will limit what types of reports can be run.  Do no use, for example, if user-level logging/reporting is required or if you wish to include charts covering VPN history or application detail.

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Technical Note : How to disable unrequired entries being logged to the traffic log