I've been trying to find an explanation for this for awhile. In our ESP our email file size is, let's say, 40kb. However, when we receive it in our inbox the email comes anywhere between 30-50kb higher. I have some that jump from 47kb to 95kb, etc.

Does anyone know why this is occuring? Is it something the ESP is adding? Do different email clients add file size?

The only other thing that is being added is user data, as in first name and email address.