MMS as a whole has a limit on file size that can be sent. Part of what determines what limit will be used is depending on the networks it is going to and the devices that we connecting with the content.
T-Mobile article says that their limit is 1 MB. I have sent larger messages but not by much. Usually 1.1 or so.
Verizon max size is 1.2
There will need to be a cap on this for compatibility reasons. MMS also needs to be able to support dumb phones as well. Yes, some people still use them.
If you want to send full quality videos, you will either need to email the file if it is smaller than the limitations of email attachments, or use something like Google photos to share online.