The majority of carriers enforce a 1 MB size limit on MMS attachments. This, unfortunately, means that the quality and resolution of media like videos need to be decreased (sometimes substantially) so that they can fit under that limit, but still be useful to the recipient.
Since this restriction is already in place through carriers, it is not something we will be able to work around or improve in the app. If you want to send full-quality images, we recommend using Google Photos and sharing a link to the images.