I agree compression is probably being used but your calculation of size is off. I imagine they prefer to keep everything shot (which is where the magic deleted scenes come from).

According to my fading memory from film class many years ago regular film would have a shooting ratio of 6:1. Documentaries were lower.

Even lossy compressions are going to have a hard time with that much footage.

I think the way i'd store one movie is to put it on hard drives, plus one extra hard drive that has a parity calculation of the other drive (RAID 5). then you could lose a storage disk and still recover.