FortiAnalyzer can receive logs and Windows host events directly from endpoints connected to EMS, and you can use FortiAnalyzer to analyze the logs and run reports.
The default amount of disk space allocated when a FortiGate is added to a FortiAnalyzer is 1GB (1000MB).   
This article describes how to increase the amount of disk log quota.

The quota can be increased as follows:

1) Select Device Manager.
2) Select the appropriate ADOM.
3) Select 'All FortiGates'.
4) Under Device Name, select the specific FortiGate and select 'Edit'.
5) Change the value for disk og quota (minimum value is 100MB) to a higher value.
6) The maximum amount allowed available will be displayed.

To change the default value for when a new FortiGate is added, go to the CLI on the FortiManager and enter the following:
# config system global
    set default-disk-quota xxx              <----- 'xxx' is the amount of disk space in MB.

Related Articles

Technical Note: FortiAnalyzer disk quota allocation per device

Technical Note: Extending log disk quota storage on FortiAnalyzer

How to understand Quota Configuration FortiAnalyzer 5.4 and 5.6