According to a research paper presented to the American Chemical Society by Theo Clark, a visiting chemistry professor at Truman State University (Kirksville, Mo), oranges that are not organically grown are larger and of deeper color than organically grown oranges. The organically grown oranges contain 30% more vitamin C, even though they were about half the size of the conventionally grown fruit.
Conventionally grown fruit is grown using nitrogen fertilizers that cause more uptake of water, creating larger fruit. The fruit, however, has a lot of water, but not as much nutrient. Dr. Clark says that this is speculation, but it is one possible explanation.