一个奇怪的问题？！

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

struct student
{
string Name;
int age;
bool isBoy;
};
int main()
{
student xiaoming;
xiaoming.Name = "小明";
xiaoming.isBoy = true;
xiaoming.age = 19;

student *pStu = 0;
pStu = &xiaoming;

int* pInt = 0;
pInt = &xiaoming.age;
bool *pBool = 0;

pBool = &xiaoming.isBoy;
string* pString = 0;
pString = &xiaoming.Name;

cout << pStu << endl;
cout << pInt << endl;
cout << pString << endl;

cout << sizeof(pStu) << endl;
cout << sizeof(pInt) << endl;
cout << sizeof(pString) << endl;

cout << sizeof(xiaoming) << endl;
cout << sizeof(xiaoming.age) << endl;
cout << sizeof(xiaoming.isBoy) << endl;
cout << sizeof(xiaoming.Name) << endl;

return 0;
}

执行结果为：（我的环境是XPSP3，VC++2008）

sizeof(xiaoming)=40；sizeof(xiaoming.age)=4；sizeof(xiaoming.isBoy)=1；sizeof(xiaoming.Name)=32

神奇了，如果按以前学过的知识sizeof(xiaoming)=12才对嘛？？？？？？？？？