More water in a partially filled tank is the only way you could get better mpg with a heavier vehicle.Gonavy-good points about the small amounts that could be dissolved but the water wouldn't have to be dissolved or emulsified in the gasoline,it could be a simply mixture(lotta shaking vibrating going on in a tank).
Several weeks of condensation in a half filled tank would probably produce a few ounces of water(I'm not up to actually checking how much water is in ,and would drop out of 6 gallons of air-not a lot).I'm guessing that several wks worth of cycles might produce 6-12 ounces of water in-under- 700 ounces of gasoline in a 12 gallon tank.This is 1-2%-not a lot.I really doubt that you could reliably detect a 1-2% change in FE;normal day to day traffic variation would cause a greater variation than 1-2% and the wt loss of 6 gallons-36 lbs-1%-should cancel it out.
I think the better FE with the full tank is just a coincidence-normal variation you would see day to day in driving.The lighter weight favors the empty tank-no way around that.Charlie