I've been on a makeMKV rip lately. But I've been then handbrake'ing them to go on my phone. A blu ray comes down to 1.8GB or so at 1080p fast settings. I could probably make it smaller.
Plex server can do that on the fly if you choose to “download” a movie to bring it offline on your phone.
For me, I want all the original bits on the server and let it encode on the fly per device if required.
But you could store 10-20x the movies if you pre-transcode them.
As long as I can’t tell the difference (which takes some trial and error sometimes) I’d rather fit more movies onto the server.