Should we be worried about [the trend toward large firms]? Many people over the years have bemoaned the death of the mom-and-pop store or the local restaurant, but I'm not sure we should be too concerned. Go visit a poor country, and you'll see that every other family has a food stand or little clothing stall, etc. That kind of "entrepreneurship" takes guts and ingenuity, but it's born out of hardship, not opportunity. The birth and death of small family businesses is one kind of "economic dynamism", but not really the kind that leads to increasing living standards or technological progress. In the U.S., it's not clear from Brookings' data how much high-growth entrepreneurship has changed.
Read the whole thing, and links therein. See also my comment here.