How is this possible?
It has to do with how Gmail counts the size of attachments versus how the Gmail API counts the size of an email message.
Using the Gmail user interface, you can attach a file up to 25 MB in size, where the size is determined by the size of the file on disk. If you attach a file larger than 25 MB in size, then Google uploads it to Google Drive and places a download link to the file in the body of your email message.
However, even if you attach a file smaller than 25 MB, by passing the need to use Google Drive, the actual email messages ends up being around 50 MB in size, because of how attachments often double in size when encoded into the format necessary for email transmission (MIME).
GMass uses the Gmail API to make mail merge inside Gmail possible, and the API methods have a hard limit of 35 MB for the total size of the email message, after MIME encoding.
Usually, when a GMass email exceeds 35 MB, it is due to either excessively large attachments or large inline images. It may appear that your attachments add up to less than 35 MB, but when a file is encoded into the format necessary for email sending (MIME format), an attachment can often double in size. For example, if your attachment is only 20 MB as you see it in the Gmail Compose window, you will likely still exceed the limit, because a 20 MB attachment equates to almost 40 MB when converted into MIME format.
Therefore, in terms of using GMass, that means your complete email message, when encoded into MIME format, cannot exceed 35 MB. That equates to roughly 17-18 MB of inline images and attached files. Note that if you’re referencing images from an external host, that doesn’t count towards the size limitation.
The GMass size limit applied to an individual email message, not all emails in aggregate. Meaning, it doesn’t matter whether you’re sending to 1 email address or 1,000, the composed MIME-encoded email cannot exceed 35 MB.
GMass Size Limit Summary
When using GMass to send large email campaigns, you’re limited to about 17-18 MB of inline images and attached files, because when encoded into MIME format, this results in an email of about 35 MB in size, which is the GMass email size limit.
If using the regular Gmail Send button, you’re limited to 25 MB of file attachments, as seen on disk, but you can still send even bigger attachments if you’re okay with a download link instead of the file being attached to the email.
Incoming Email Size Limit is 50 MB
To make matters even more confusing, the above-referenced limits apply to emails sent from your Gmail account or with GMass. The size limit, however, for emails received by your Gmail account is bigger, at 50 MB.