[Boost-users] How multiprecision optimizes allocation?

I have tested boost::mulitprecision::cpp_int and have tested simplest algorithm for int multiplication My test method is several times slower than boost multiplication because I resized product vector to size fist + size second. This allocation take most time, if I use static array on stack, it will be fast for small numbers. What is the riddle of fast comnputing z = x*y in loop for quite small x and y in Boost cpp_int?