If you sum the compressed size of all entries in a zip the result will not equal the size of the zip file. Offset Size Description 0 4 Local file header signature ... ... ... 26 2 File name length (n) 28 2 Extra field length (m) 30 n File name 30+n m Extra field Offset Size Description 0 4 Central directory file header signature ... ... ... 42 4 Relative offset of local file header.