From my experience in research, my understanding is that different animals are used for different types or levels of testing. For example, the initial testing of a drug is usually to test its cytotoxicity and immunogenicity. These tests are usually done in rodents such as rats or mice because they are abundant, easy to get, and are small. If the initial test is done using a devise that is too large for rats and mice, they are done on rabbits. Once the drug/device passes the initial tests, they are further testing for function in animals similar to humans (to predict their function in humans) such as pigs. For more advanced testing and precise data, they are done in primates.

This logic of testing is one of many branches. Depending on the product being tested, the tests can vary and so can the animals. But my point of my example is that the use of animals is carefully thought out to benefit the humans while minimizing pain to animals, especially larger and more sensitive ones by doing tests with small quantities of drugs in small animals and then moving up when needed.