Single API for Web and Mobile

If I've Single API for Web and Mobile then how can accomplish following things.

1.In web I need to display 10 fields but in mobile may be 3 fields only ?

2.Basically we will have tight coupling on DB proc side as well if there is a change(Requirement) for WEB, then it could be hit Mobile as well ?

3.Testing -> May be lot of testing for all platforms ?