Question on multiple inheritance, virtual base classes, and object size in C++

The following code prints 20, i.e. sizeof(z) is 20.

#include <iostream.h>
class Base
{
      public:
            int a;
};

class X:virtual public Base
{
      public:
            int x;
};

class Y:virtual public Base
{
      public:
            int y;
};

class Z:public X,public Y
{
};

int main()
{
Z z;
cout << sizeof(z) <<endl;
}

Whereas if I don't use virtual base classes here, i.e. for the following code : sizeof(z) is 16.

#include <iostream.h>
class Base
{
      public:
            int a;
};

class X:public Base
{
      public:
            int x;
};

class Y:public Base
{
      public:
            int y;
};

class Z:public X,public Y
{
};

int main()
{
Z z;
cout << sizeof(z) <<endl;
}

Why is sizeof(z) more(20) in the first case? Shouldn't it be 12, since Base will be included only once in Z?