How the compiler does create an object of derived class using the pointer of the base class?

For example we have the following hierarchy.

class A
{
int a;
char* b;
public a() {}
};

class B
{
int c;
chard;
public:
}

Now, lets allocate memory for derivet class B, using the pointer type of the base class (let's using malloc, because this way is more understandable)

void* a = malloc(sizeof(A)); // we use the size of the class A, which is equal 8 byte in x32 machine
A* obj = new(a) B(); // placement new syntax runs the constructor of B()

The same we can write by following way

A* obj = new B()

In this case we allocate 8 byte of memory for class B and call the B's constructor, but B need to have 12 byte of size, what happens in this case ?