How can 1080p films be so disproportionally lightweight?
DVD and Blu-Ray content makes massive use of lossy encoding.

An easy way to see for yourself is to find a dark scene in a film, and then pause it. If the only issue was a limited number of black levels, you would expect to see a stepped gradient curve from the darkest region to the lightest region. Of course, if true 24-bit color was being used, the "stepping" would be too subtle for your eye to see. But the video isn't even that good! You will most likely see large solid rectangles in the black part of the screen. This is the result of the compression algorithm "guessing" that a square is a good enough approximation, and everything within the square is close enough to the same color to just use one color.

This works well most of the time, but tends to fall apart in films that make extensive use of dramatic dark scenes.

Added from comments: Encoders "cheat" on everything they possibly can. If you can think of something new to "cheat" on that most people won't notice, you can make a lot of money licensing your codec in the video production world.