Darwin's writings indicate that such diversity of form was one factor that led to his ideas on evolution. All giant tortoises are indeed giant. There are two main carapace designs. Tortoises that occupy islands with fairly lush vegetation are large with a domed carapace. This carapace shape along with long necks and limbs helps them reach their food.
Intermediate forms occupy other islands.
Tortoises are vegetarian, eating large quantities of over 50 native plant species including prickly pear cactus and fruits, bromeliads, water ferns, and the so-called poison apple. They have enormous storage capacities, being able to store enough calories to enable them to survive the long arid season of the islands. Tortoises are slow digesters. It may take up to three weeks for food to pass from one end to the other. Giant tortoises have historically enjoyed an idyllic existence on the islands.