Docker文件大小问题

我正在用docker在linux下读文件：

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

而当我打电话给我的档案我得到：

 $ 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: - 

统计调用的st_size是8993。
任何想法，为什么docker行为像这样？