I have a DVD hard copy, but no DVD player. Anyway, I found the DVD online and downloaded it, only problem is it's a 7GB mkv. Is there a method to shrink it down by around 6GB?
If it's in an uncompressed encode you could trancode it to HEVC or AV1, which should cut the size by 1/3. Though some older phone might not be able to play AV1.
Alternativly you could convert it to a lower resolution.
I personally use ffmpeg on the command line, but I think there are some apps that add an UI for easier use.
Plex media server can also do some pre-set conversions. Either way you probably want to do that on a computer, it take a couple of hours to convert a movie even on my i9 CPU.