In updates, there are two extremes: making everything available with each update ("full updates"), and making only the changes available ("incremental updates").
Spybot takes a middle approach, having two sets of files now, one for changes and recent stuff, one (actually divided into a bunch of updates) for the rest.
And even though the updates are smaller now (about one third than what it was a few months ago), they're in a way that makes sure that whether you skip one update or not, you'll always receive everything.