RCS is the newer protocol that allows for sending larger files like most videos (but in order for that to work, the other person you send it to also has to use RCS). If you're just using plain MMS (the very old standard), the maximum filesize you can attach is usually around 1 MB (which would be a very very short video, or a really low resolution video, or both).
It's usually easier to upload your video to your Google Photos in the cloud (or to YouTube), and share the link privately with the recipient.