Varicose veins and spider veins are similar in that they are both caused by the weakening of blood vessel valves and walls. The difference is their size and location. Spider veins are the tiny capillaries that are situated near the skin. When the valves in these blood vessels stop working efficiently and blood pools, you see small "webs" of purple, blue, and red. Spider veins can swell but this is usually subtle because the veins are so small. Varicose veins occur in blood vessels that are larger and slightly deeper beneath the skin. You might see some bluing or other discoloration of the vein but, primarily, varicose veins look swollen and twisted as they protrude outward. Varicose veins can also become symptomatic and painful, whereas spider veins typically do not cause many symptoms beyond their appearance.