I'm trying to send an email through Mail.app (which has a limit of 20MB for attachments). My rar'd attachment is 19.3MB, but Mail says it can't send it because the size is 26.4MB
Without going into unnecessary details, the bottom line is that email is such an old technology that it sends files in a way that makes them bigger in transit than the size they are on the sender and recipient's computer. So it'll be the same size when you receive it on the other end, but in transit it'll be slightly larger.
If you want details on why this is the case, then read this.
