Your disk capacity for kintone and other cybozu.com services (e.g., Garoon) is the number of licensed users multiplied by 5 GB. The number of licensed users does not include guest users.
For details, refer to the product website.
Pricing (Only available in Japanese)

Your current disk usage is calculated based on data such as the data saved in the services (e.g., attachments) and configuration information.

The disk capacity for your entire environment and the current disk usage are found in the Data Usage row on the "Service Usage" screen in Users & System Administration.
To access Users & System Administration, you need the Users & System administrative permission.
Opening the Users & System Administration Screen

The Data Usage row shows the following information:

• The Current column shows the current disk usage.
• The Limit column shows the disk capacity for your entire environment.
  The additional disk space you have added is displayed in parenthesis.