Google will often convert it to a gdrive thing instead. So you're not sending the file, just a link to the file uploaded somewhere. I'm not sure what heuristic it uses, but sometimes when mailing photos like half of them are included in the mail and half automagically uploaded to gdrive instead.

That sounds like some kind of weird google interface issue. Maybe try using IMAP or POP or whatever standard they still deign to support.

Anything over 25mb goes out via gdrive if memory serves. That’s at least how it used to be.

email providers have limits for size. Modern files are huge.