Hello Sparrowsavs,
If this is occurring with just one person, then it could be due to the person's video resolution of the camera used to take the video.
If it's not related to the user's camera, since the message is going through MMS with your wireless carrier, your carrier may be setting the size limit.
Check out the information in the following article:
"Your carrier may set size limits for attachments; iPhone may compress photo and video attachments when necessary."
Send and receive photos, videos, and audio messages on iPhone - Apple Support
If you tap the download icon next to the message does it play larger?
See those steps listed here to see if they help: Send and receive photos, videos, and audio messages on iPhone - Apple Support
"Share, save, print, copy, or delete a message or attachment
In a conversation, do any of the following with attachments:
- Share, save, or print an attachment: Tap the attachment, then tap ."
If the issue persists with all MMS messages, but not with videos sent via iMessage, then reach out to your wireless carrier for more details.
Kind regards.