[GoatMother]

Straight from Ted Price, I'll take his word over some Neogaf forum member anyday:

As we moved into the final week of finishing up the game we made two big changes which dropped the size of the final disc to just over 16Gb – still pretty large nonetheless. What were the changes? We removed PAL movies and further improved our data compression. It turns out that the NTSC movies (not HD, but NTSC just in case Lachamania is reading this) could be converted on the fly to look really good on PAL TVs and therefore we saved space by removing them. It's something that was a pleasant surprise. Why did we bother when we had so much space to work with? It simply saved us time when burning discs. When you're making revs every day and distributing them to a large test team, saving burn time is crucial. We could have left the PAL movies on disc and not improved compression to keep the size around 20Gb but that would have been inefficient and a little disingenuous. We don’t want to pad the disc just to get it to a certain size. Whatever the game needs is what we put on the disc. Still, having a lot of space on Bluray means that we’re including things we wouldn’t have been able to include if we had had to use dual layer DVDs: higher res game assets and more of them, HD movies, higher fidelity sounds, more dialogue, all languages on one disc, etc.

And of course, as others have pointed out in the many threads on this topic, the debate over disc size is a little silly. What really matters is whether your experience in the game is fun.

http://blogs.ign.com/Ted-Insomniac/