The phototransistor should conduct more electricity when light hits it. If it is hooked up as in the diagram I referenced:
+5v ----- (c)Phototransistor(e) ----- [A0] ----- Resistor ----- Ground
the voltage across the resistor should go UP as the light increases. I don't understand why you are getting the opposite effect unless you have wired it differently. you are getting the opposite effect then I think the wiring is done differently.
Try different values fro the resistor to see what effect that has on the output.