Association does not imply causation. Urbanization may be contributing to higher temperatures and easier access to food, but attributing increased spider sizes to urbanization is as foolish as asserting that ice cream causes people to drown more often (which as been suggested in the past). Both fall victim to confounding. Ice cream sales increase when the temperature rises, and people swim more when it's warmer, so more people are likely to drown. By the same logic, urbanization may be contributing to higher temperatures, but urbanization itself isn't necessarily the cause of increased spider sizes.