Maybe im reading the OP incorrectly, and i apologize if i am. The only thing that takes up more space on the blur-ray is if the movie were longer.
You can make a movie that has special effects so realistic it will melt your brain, but it will still take up the same amount of space per minute of movie. So any movie at 1080p/7.1ch will take up the same amount of GB/hour, regardless of the quality of the image.
You're right, but wrong at the same time. First, special effects have no bearing on disk space used, you're quite correct in that. However, image quality is directly related to GB/hour, but again, even for 2 movies with the exact same image "quality", GB/hour can differ.
It's wrong to base a movie's size on length alone. A lot of different factors come into play. First is the bitrate encoding. If you're using CBR (constant bit rate) , which no one uses because it's plain dumb, yes the movie will have a static GB/hour for a given bitrate. The only variable then will be the encoder's chosen bitrate. Bitrate is basically the amount of information used to encode a given timeframe. It's usually listed as mbps (megabits per second) or MBps (megabytes per second). A movie with 40 MBps bitrate will of course be twice as big as a movie with 20 MBps bitrate for the same length of time.
In a CBR scenario, if you use too low a bitrate, your calm scenes will look fine while your action scenes will suffer greatly and have tons of compression artifacts. If however you use too high a bitrate, your action scenes will look fine, but your movie will take up tons of extra space that is getting wasted during the calmer scenes, since the quality won't improve for those particularly.
However, a VBR (variable bit rate) stream will have lower bitrate during more static scenes (think two people sitting down simply talking in a quiet room) and a higher bitrate in fast action scenes (large armies meeting on the field on the battle for what is an epic struggle between man and god!). This will give you a smaller movie, with the same perfect quality during action scenes and the same quality during calmer scenes. The bitrate will adjust automatically so that there's just enough to not have compression artifacts on screen during either scene, but use the less space possible. This is how movies are encoded these days.
So it's wrong to say length of a movie is the only variable. The amount of action on screen (changing scenes, fast paced action, etc..) also can result in a larger movie (more scenes at higher bitrate averages) for the same given length. Then there's bitrate targets that the encoder sets. Higher bit rate targets = more space used. Lower bit rate target = less image quality.
Of course, there's the codec (some can compress more effectively than others, giving you the same quality at lower bitrates) that matters too, but let's not overcomplicate things. Blu-ray mostly uses H.264 anyhow.