In fact .zip file contains file header information twice: before every compressed file (local file header) and at the end of the .zip file (central directory).

In fact .zip file contains file header information twice: before every compressed file (local file header) and at the end of the .zip file (central directory). It's possible to omit file size in local file header, but most most compressing utilities doesn't use this option. So most .zip files could be extracted without seeking to the end.