On my MBP specced as in my signature, the update was 198mb. This seems to be the minority based on other 10.5.3 threads, as others have said their downloads were 420mb. So far, there doesn't seem to be any clear pattern in download sizes based on the specs people have given. What could be the factor that makes these downloads so different in their sizes? Here's the first hypothesis I'll throw out. Feel free to falsify/refute with evidence: I'm using Time Machine with a Time Capsule. There are a few fixes noted under the TM/TC section of Apple's notes. Could part of 10.5.3 have been included on a previous TM/TC update? If so, it would make sense that it wouldn't update that part twice, right? If you're going to reply to this thread, tell us as much about your specs as possible (as well as how big your download was, of course), so we can try to figure out why these download sizes are so different. Feel free to add your own hypotheses as you see patterns in peoples' specs, too.