Many large companies worry about investing in a product or service provided by a small company. They fear you don't have the staying power. We are only 10 employees but we have many customer success stories including some very large companies. Looking for unique ideas on how to answer this question in a way that plays more to our benefit.
Always play to your strengths and make it be about them (not you). Be prepared to discuss why going with a small firm gives them an advantage. (like Shawn said ... excellent answer)