Chapter 23 WAN Optimization, Web Cache, Explicit Proxy, and WCCP : Web caching and SSL offloading : Changing web cache settings : Max cache object size
  
Max cache object size
Set the maximum size of objects (files) that are cached. The default size is 512000 KB and the range is 1 to 4294967 KB. This setting determines the maximum object size to store in the web cache. Objects that are larger than this size are still delivered to the client but are not stored in the FortiGate web cache.
For most web traffic the default maximum cache object size is recommended. However, since web caching can also cache larger objects such as Windows updates, Mac OS updates, iOS updates or other updates delivered using HTTP you might want to increase the object size to make sure these updates are cached. Caching these updates can save a lot of Internet bandwidth and improve performance when major updates are released by these vendors.