It's because BigCo's tend toward decisions with dumb outcomes, while SmallCo's still benefit from the strategic direction of their founders. Although not a hard and fast rule, if you take a good look at where innovation occurs you'll often find the most successful products of big tech companies (after the initial one that made them big in the first place) came from acquisitions. Just look at Google's case: Android, YouTube, Maps, etc.

Sometimes the aquisition doesn't pan out as planned, or they were just after the talent or to snuff out a potential competitor / snag its customers (like Postini), or it was a dumb move in the first place and the numbers finally bore that out. BigCo's don't usually have the same determined, long-term dedication to their acquisitions as the people who the founded them, so you also see premature shedding of ventures that could have a ton of potential over time.