Docker文件大小问题

Question

我正在阅读docker with linux下的文件：

fp = fopen(_path, "r");  
readCnt = fread(buffer, 1, payloadLen, fp);  
//payloadLen = 10240  
//readCnt = 8993  

当我在文件中调用stat时，我得到：

$ stat /tmp/vc.bin  
File: /tmp/vc.bin  
Size: 10240         Blocks: 24         IO Block: 4096   regular file  
Device: 35h/53d Inode: 96          Links: 1  
Access: (0644/-rw-r--r--)  Uid: ( 1000/builduser)   Gid: ( 1000/builduser)  
Access: 2017-03-27 06:57:19.272826554 +0000  
Modify: 2017-03-27 06:57:19.272826554 +0000  
Change: 2017-03-27 11:33:42.125708017 +0000  
Birth: -  

来自stat call的 st_size是8993.
有什么想法为什么docker的行为像这样？