Attachment file size

Hi i have a zip file with size7.2MB(approx 7549747 bytes). In thunderbird mail client(linux version) i created a mail with this file as attachment and saved that mail as eml file in my hard drive. After that i read that eml file using JavaMail API, but it gives the attachment size as10159900 bytes. Why is it so?. While creating mails will it add any extra informations?. How this much size difference?. Please help me.

Thanks.

[457 byte] By [MAYAa] at [2007-11-27 5:45:40]