Need of small charge in definition of electric field?

[Raj Batra]

Why do we need infinitesimally small charge in definition of electric field? Since the test charge cannot exert force on itself, F on test charge will not change whatever the value of test charge q is. So, F/q will be same for any value of test charge. Then why do we need this limit of infinitesimally small charge?

 

[mfb]

A larger charge will influence the charge distribution in the environment, and therefore the field at its position.