iCloud and OS X Server: Limits for Contacts, Calendars, Reminders?
The data repository for Calendar service is located at /Library/Server. Inside that directory is a folder containing data specific to the Calendar service at /Library/Server/Calendar and Contacts.  Within that folder is a Config folder that has the main configuration file for the caldav daemon called caldavd-system.plist.

The limits are based on your configuration and physical capacity.  Outside of that users can have as big a Calendar and Contacts as they want/you want them to have.  Apples limits are arbitrary for use of their service.

EDIT There is a difference between limits and compatibilities.

BONUS If you want to learn how to set up your own limits try here.