About get the information of a folder

Hello, everyone!
This is Echo. I want get the size, file numbers of a folder. i made
following code. It works for most folders. But for larger folder, there is
problem.
The problem is: if I choose a larger folder, such as C or other drives, the
result is different to the results obtained by windows OS. For example,the C
dirve on my PC, if i select all the folder and file in C and right-click
"properties", windows OS gives the result is 133,055 files, 18.0G. But, my
code gives result is 148,368 files and 27G.
I cannot find any error in my code. Can you can help me? Thank you in
advance!!!

unsigned long SizeofFolder(CString path, DWORD& gb, DWORD& mb, DWORD& kb)
{
   DWORD num=0;
   gb=mb=kb=0;
    CStringArray dirs;
    CString searchname;
    CFileFind find;
    dirs.Add(path);
    BOOL bRet;
    while(dirs.GetSize()>0)
    {
       searchname = dirs[0] +"\\*.*";
       dirs.RemoveAt(0);
       bRet = find.FindFile (searchname,0);
       if(!bRet)continue;
       do{
           bRet = find.FindNextFile ();
           if(find.IsDots ())
               continue;
           if(find.IsDirectory ())//This is a folder
           {
               dirs.Add (find.GetFilePath());
               continue;
           }
           else//This is a file
          {
     num++;
     kb=kb+find.GetLength();//bytes
     if(kb>=1048576)//1M byte
                 {
       DWORD i=kb/1048576;
         mb=mb+i;
           kb=kb-i*1048576;
       }
       if(mb>=1024)//1G byte
       {
           DWORD i=mb/1024;
          gb=gb+i;
          mb=mb-i*1024;
      }
           }
       }while(bRet);
    }
   find.Close();
    return 1;
}