Thunderbird, like any mail client can only send mail in the approved format. In the case of mail, that is plain text. The result is that attachments, all of them, images documents and Zips have to be encoded in the message using MIME. For binary formats like Zip this leads to around a 30% increase in the size of the attachment. So no there is no way to stop the increase in size and it is a requirement of the email formatting rules. The use of Filelink was introduced a number of years ago to enable the placing of the binary files onto a cloud based file service so you do not need to go through the file size growth that MIME encoding brings with it. https://support.mozilla.org/en-US/kb/filelink-large-attachments My understanding is that Thunderbird no longer ships with a built in filelink provider but there are a number of providers listed in the addons for Thunderbird. https://addons.thunderbird.net/en-US/thunderbird/tag/filelink Note that the link is best opened from the settings in Thunderbird as it limits the available providers to those that have support for the version of Thunderbird you are using. Otherwise you get to see them all.