Its nothing but logic.
Big companies are run by hundreds and thousands of people who are told what they have to do and there is little room for experimenting.
On the other hand, a startup has to try anything and everything that can beat the big guys and give them more customers.
Innovation has various levels. Check
A Message from CTQ Media and
http://www.techmonitor.net/techmon/0...innovation.pdf
The only thing that gives small companies an edge on big companies is the level of experimentation they could do, which results in quantum leaps of differentiation at product and service level, which is called innovation.