This is a common issue with text messaging when higher resolution media content (photos or videos) are sent as attachments. Every carrier has its own, arbitrary file size limit for attachments so when a photo or video file size is too large, they get automatically scaled down on-the-fly so they're small enough. Just some optional work-around suggestions:
-- Try experimenting with other text messaging apps as some are better at negotiating through that file size issue. Don't expect too much though, the problem isn't a phone issue nor is it an app issue, it's how the carriers transfer file content.
-- Scale down any photos/videos you send over text messaging yourself. Your carrier will have their file size limitation numbers posted somewhere in its site and then you'll need to be sure scale down and/or use a high enough compression to compromise the resolution with file size limitation.
-- Use email messaging instead. File size limitations are much more acceptable, most email services allow up 20-25 MB or more. Again, check your email services web site, they'll have their email limit numbers buried in a spec page somewhere.
-- If you have your phone's photo library synced up with an online storage service, like Google Photos, Google Drive, MS OneDrive, Dropbox, etc. just get the sharable link for any photo/video you want to share and just text message your recipient that link. This way it's not a matter of the file size of the actual file at all.
-- You can also use a temporary, online file transfer service like Firefox Send, which despite its name works with any web browser.
