A game using 25GB/50GB on a Blu-Ray disk doesn't mean that there is 25GB/50GB of unique gaming data on that disk. When game developers moved from memory cartridges to optical discs the most obvious problem with the format was how slow optical discs are; the maximum transfer rate is bad enough, but with how slow the latency and seek time are whenever you switch the area you're reading from your performance takes a massive hit. From my very limited understanding of Blu-Ray drive in the PS3 it has a slower transfer rate, higher latency and worse seek time than an inexpensive DVD drive.
Now, there is a trick to prevent load times that has been used since CDs were first choosen as a format. If you have enough space all you do is take all of the content (textures, models, sounds, etc.) that are related to a particular level, zip them up together and store them on disc as one file. The result is that you can load the entire level about as quickly as is possible using that format. The downside is that your data gets duplicated; in a 20 level game you can (hypothetically) have a lot of data (models, textures, sounds) duplicated 20 times.
(I appologize for the simplified explaination)
