Как sizeof считает размер?

#include <iostream>
 
class ImplClass //áàçîâûé êëàññ
{
public:
   void do_work_1()
   {
      std::cout << "Doing some work 1" << std::endl;
   }
   
   virtual void do_work_2()
   {
      std::cout << "Doing some work 2 in base implementation class" << std::endl;
   }
};
 
class Derived : protected ImplClass // protected inheritance should be used as implementation
{
protected:
   virtual void do_work_2() // ïåðåîïðåäåëåíèå áàçîâîãî êëàññà
   {
      std::cout << "Doing some other work 2 in derived class" << std::endl;
   }
public:
   void perform_operation()
   {
      // calling some implementation methods.
      // Usually, this is achieved using PImpl idiom (except some moments), but here we have a directive
      do_work_1(); // calling method of base class, which is protected for us.
      do_work_2();
      
      // Doing some important work with added class members.
      state_ = 42;
   }
   
private:
   int state_;
};
 
struct DerivedStruct : protected ImplClass
{
// public - îïðåäåëåíèå íå íóæíî. 
   void perform_operation()
   {
      do_work_1();
      do_work_2(); // not overriden here
   }
};
 
 
int main(int /*argc*/, char ** /*argv*/)
{
  /* Derived d;
   d.perform_operation();
      DerivedStruct ds;
   ds.perform_operation(); */
   
   
   std::cout << "sizeof(Widget)" << sizeof(DerivedStruct) << std::endl; //4
   system("Pause");
}