Actual, a lot of economic activity comes from small businesses: about half the US GDP is generated from companies with less than 500 employees.
http://en.wikipedia.org/wiki/Small_business#Contribution_to_...
http://en.wikipedia.org/wiki/Small_business#Contribution_to_...
Here's the full PDF of that source: http://www.sba.gov/sites/default/files/FAQ_Sept_2012.pdf
Also, while they're a government agency and cite their sources, their job is to advocate for small businesses. Does anyone else have more sources to look at?