A petabyte is the equivalent of 500 hard drives, each with a capacity of 2 terrabytes (a terrabyte to 1,000 gigabytes!). That's the size of a 32 YEAR long mp3 file!!
The movie was shot in 3-D and rendered in one of the most powerful data centers in the world. Its computing core adds up to 40,000 processors and 104 terabytes of RAM. It would be realistic to assume that a significant portion of the film's $300 million budget went to storage space.