I'm not at home to check this, but I recall something like 30% od our gdp being in the FIRE sector, which is certainly dominated by large corporations. Consumption is another huge part, and I imagine most of that is done at wal-mart and other major chains. The other prop holding up our economy is massive state investment. If it's not the case now, the crowding out of small business - the "tendency toward monopoly" is long-standing trend that I don't see slowing.

Also one caveat with the "small business" economic numbers is they're usually not what we would normally think of as "small business." They're often counting all firms with less than 50 employees IIRC.