I suspect partly size but mostly tracking. Images in email have been around since 1992 (RFC341 Multipurpose Internet Mail Extensions https://tools.ietf.org/htm

I suspect partly size but mostly tracking. Images in email have been around since 1992 (RFC341 Multipurpose Internet Mail Extensions https://tools.ietf.org/html/rfc1341 https://tools.ietf.org/html/rfc1341 ) and I don't remember when downloadable images got added to "text/richtext" but the concept of downloadable content is already there in section 7.