San Diego Zoo is not the largest zoo in the world. The San Diego Zoo Safari Park, the much larger, related park to the north is 1800 acres, 1000 of which are available to the public. The other 800 acres are home to the Beckman Center, officially known as the Institute for Conservation Research, and the Harter Veterinary Medical Center, and areas which house off-habitat animals. There are about 3,000 animals at the Safari Park. This would give it a score of 1,800x3,000=5,400,00.
The San Diego Zoo, the original, related zoo, by comparison, is only 100 acres. The San Diego Zoo, however, has many more animals, over 14,000, but obviously housed in much smaller spaces. This would give it a score of 100*14,000=1,400,000. Adding both parks together, which is very doable in 2 days (some masochists do them both in one day) would be a combined score of 6,800,000.