new and delete

Hi,
Below is a small code about memory allocation and deallocation.

#include <cstdlib>
#include <iostream>
using namespace std;

class X {
public:
  void* operator new(size_t sz) throw (const char*) {
    void* p = malloc(sz); //LINE1
    if (p == 0) throw "malloc() failed";
    return p;
  }

  void operator delete(void* p) {
    free(p);
  }

};

class Y {
  int filler[100];
public:

  // two arguments
  void operator delete(void* p, size_t sz) throw (const char*) {
    cout << "Freeing " << sz << " byte(s)" << endl; //LINE2
    free(p);
  };

};

int main() {
  X* ptr = new X; //LINE3

  // call X::operator delete(void*)
  delete ptr;

  Y* yptr = new Y;

  // call Y::operator delete(void*, size_t)
  // with size of Y as second argument
  delete yptr; //LINE4
}

My questions are:
 When LINE3 is executed, LINE1 is executed. How is the variable sz
assigned sizeof(X)?

The output of the code is:
Freeing 400 byte(s)
When LINE4 is executed, LINE2 is executed. How does sz get the value
400?

Thanks a lot.

Jack