Cactus leaves are modified, because they're characterized by smaller surface areas. Those cactus plants that have modified leaves have them as thorns, spines, spikes, quills, prongs or bristles. So the cactus leaf has a narrower, thinner look than the leaves of deciduous plants.
The reason for this modified look is the effort to keep water loss to a minimum. Water is lost at a lower rate and in smaller quantities over a smaller surface area. Additionally, the narrower, thinner look helps the modified leaf to direct any available moisture on its surface to drip to the ground. On the ground, the drops seep into the soil, where they can be taken up by the cactus plant's network of fibrous, shallow roots.