如何确定虚拟基类及其派生类的大小

#include <iostream>
using namespace std;
class base1{};
class base2{virtual void show(){}};
class test1{    };
class test2{virtual void show(){}};
class derv1:public virtual base1{};
class derv12:public virtual base2{};
class derv2:public virtual base2, public test1{};
class derv22:public virtual base2, public virtual test1{};
class derv222:public virtual base2, public virtual test2{};
int main()
{
    cout<<"sizeof base1 = "<<sizeof(base1)<<endl;
    cout<<"sizeof base2 = "<<sizeof(base2)<<endl;
    cout<<"sizeof derv1 = "<<sizeof(derv1)<<endl;
    cout<<"sizeof derv12 = "<<sizeof(derv12)<<endl;
    cout<<"sizeof derv2 = "<<sizeof(derv2)<<endl;
    cout<<"sizeof derv22 = "<<sizeof(derv22)<<endl;
    cout<<"sizeof derv222 = "<<sizeof(derv222)<<endl;
}

输出：

sizeof base1 = 1
sizeof base2 = 4
sizeof derv1 = 4
sizeof derv12 = 8
sizeof derv2 = 8
sizeof derv22 = 8
sizeof derv222 = 12

我理解以下输出：

sizeof base1 : 1 => empty class , size is 1. result is OK.
sizeof base2 : 4 => since class has a virtual function, it adds 
                    a vitual pointer in each object of the class
                    ( 32 bit machine) , so added 4 bytes. Result is OK.
sizeof derv1 : 4 => base1 doesn't have  virtual function but since 
                    derv1 is virtually derived from base1 ,added 4 bytes. 
                    I think that each virtual base class
                    added a  pointer in object so i think Result is OK.     
sizeof derv12: 8 => 4 bytes( from virtual pointer) + 4 bytes
                     (virtual base class ) = 8 bytes. Result is OK.

我的困惑开始于以上输出之后

sizeof derv2  : 8 => 8 bytes( virtual pointer + virtually base class)
                    from base2 + 1 byte from test1 => 9 bytes and adds 3 padding 
                    bytes gives 12 bytes (should print). 
                     why "sizeof (test1)" is not adding in the output ??
sizeof derv22 : 8 => 8 bytes( virtual pointer + virtually base class)
                    from base2 + 4 byte (virtually base class) 
                    from test1  => 12 bytes (should print)
                    why 4 bytes from  test1 (virtual base class)  is not added 
                    in the output?? In the  size of(derv1 ) also has
                    a virtual base class( base1) and out put is 4 bytes means
                    they added 4 bytes in the output. 
sizeof derv222: 12 => 12 bytes( virtual pointer + virtually derived)
                      from base2 + 8 byte( virtual pointer + virtually derived)
                      from test2  => 16 bytes (should print)

我是不是遗漏了什么，比如这些尺寸取决于系统还是其他什么？？