I also suggest looking up how to pass by reference (pass variables by reference), because it is much more efficient, and it doesn't create new memory.
Another thing: you call it a "method", and that's not correct. Java has methods, C++ does not. This is a class you are modifying.
here is the prototype for passing by reference to get you started:
bigint operator*(const bigint&); //also look up constant correctness
Also, I would consider ONLY creating addition/subtraction functions, since multiplication/division is only an extension of them, and use those in multiplication/division processes. Of course, this only somthing to consider.