Think of it this way - a typical well-authorized DVD is about 7-9 megabits per second. A well-authorized Blu-ray that isn't too compressed is 25-35 megabits per second, so about 4 times the bandwidth of a DVD. DVD can hold up to 8.5GB while a Blu-ray can up to about 45GB. So a Blu-ray at 1080p can only hold about an extra hour over what a DVD at 480i can hold without too much compression. Hence for TV shows like Trek, they're able to fit an extra episode onto a Blu-ray disc. Any more, and you get compression artifacts.
Think about that 20-30 Mbps bandwidth next time you're watching Netflix. You're watching HD that is compressed to about 4 Mbps. That's one of the many reasons physical media isn't going anywhere for a long time. Very few internet providers can even supply Blu-ray-quality speeds, and if they did, they'd throttle you to prevent you from using that much juice.