I did compare the contents of FileInfo.Size returned by several stat functions. The resutle of the tests varies depending on the utilized methodes.

testcase 1

fi, err = os.Stat(fqfn)
fi.Size contains a value, that does not match the file size.

testcase 2

fi, err = os.Lstat(fqfn)
fi.Size contains a value, that does not match the file size.

testcase 3

file, err := os.Open( fqfn )
fi, err = file.Stat(fqfn)
fi.Size contains a value, that is correct!

What version of Go are you using (go version)?

1.9.2

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

windows, amd64
Linux, amd64

What did you do?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

What did you expect to see?

The value of the Size property should be the same regardless the utilized method. In all tree cases a FileInfo object is returned, that should describe the referenced file correctly.

In my test scenario I did create a text file containing one single byte. All three test cases should return '1' as size.

What did you see instead?

When calling the stat methods I get the following results for a 1 byte file:
2018-01-20 17:09:56, FILE "Neues Textdokument.txt"
Size: 4953040, Neues Textdokument.txt <<-- os.Stat & os.Lstat
Size: 1 <<-- file.Stat
2018-01-20 17:09:57, FILE "Neues Textdokument.txt"
Size: 4953040, Neues Textdokument.txt <<-- os.Stat & os.Lstat
Size: 1 <<-- file.Stat