Something like a video file sent via texting is a MMS, and some carriers scale down MMS videos to accommodate that different text services just aren't completely compatible with each other. If your girlfriend is using an iPhone for example and you were using the default texting app on your S7 than because of the difference in services the video was scaled down. If both of you were using the same texting app, WhatsApp for example does not scaled down videos, the chances are the video would have been left as is. But at this point since you've already deleted the original there's not much you can do to get it back. Scaling up a video file that's been previously scaled down isn't a viable option, you can't restore lost resolution. If that video is really important there are file recovery utilities that can retrieve deleted files, but note the more you've your phone since deleting that file the greater the chance it or parts of it have been 'rewritten' over in your phone's internal storage.
If you have an automatic backup/syncing service like Dropbox or OneDrive already set up, that video could possibly still be in your online backup?
