In my experience, this is super common. Lots of average computer users send video clips, uncompressed 48 megapixel photos from their cell phones, PowerPoint presentations with embedded videos and 48 megapixel images, pdfs, and on and on. At the company where I work now several people have a 1MB gif of the company logo that they put in their signature.

Combine that with Gmail telling users for years that they had plenty of space, so there was no need to ever delete anything, and people believed it. Now you have a decade of emails with giant attachments or embeds that may or may not ever get looked at, and that keeps growing.