it depends on how may content revisions are stored at the sepm, if sepm can create delta, then it will or else it will send out the full.zip to client. how many content revisions are stored at SEPM? whats the current def at the client?
if you are storing 30 revisions in SEPM, then SEPM has 10 days of defs and can send out delta,
if the client is older than 10 days, then its full,zip