A big factor is also the lack of predators. To help avoid predators, it helps to have good eyes and the brains to process what you see. If there are no predator

A big factor is also the lack of predators. To help avoid predators, it helps to have good eyes and the brains to process what you see. If there are no predators, then other factors exert their influence. Large, complex organs consume lots of energy and "harmful" mutations that shrink them will accumulate because the lack of energy on prehistoric Mallorca makes that an advantage.