- "if the client has been offline more than 15 days, it will download a full SEP 12 Virus Definition file (approx. 250MB)"
This depends on how many definition revisions you are set up to store on SEPM - 15 days of updates would be 45 definitions - during this scope the SEPM can created delta definitions so that those clients do not require full defs. Client having defs older that those 45 revision stored on SEPM will require full.zip package.
If needed you can increase the amount of definitions kept on SEPM - keep in mind this action has its own consideriations - like free space available on SEPM