Apple uses iMessage protocol, and they don't impose a limit. They won't allow anyone else to use the protocol, and the texting protocol is older than cellphones, so it's not going to be changed. There are 3rd party messaging apps that allow larger attachments, but both people have to be using the same app. (I believe you can send larger pictures on Hangouts, but I don't use it, so I can't be sure. I think B. Diddy does, though.)