SEP Virus Definition need download full size (250MB) if client offline > 15 days, is this true? How to minimize this?

We're told that "if the client has been offline more than 15 days, it will download a full SEP 12 Virus Definition file (approx. 250MB)", we have many clients rollout to remote sites, it will be big impact to network bandwidth if SEP cannot download just incremental size of file?

Have anyone had this same issue? Thanks greatly!

Regards, Xiaoqi