It all seems to make sense to me
Assuming fuel has the same density as water (I know it's actually a bit less, but I can't be bothered to look it up :P ), you're looking at about an extra 1kg of weight for every litre of fuel. So a full tank of fuel (45-50l) is the equivalent of having a small-ish passenger in your car the whole time.
It's a pretty logical (and correct) assumtion that the less your car weighs the more economical it will be. So the less fuel you put in your car, the better the fuel economy.
Obviously, you've got to find a happy medium. Putting 5l of fuel in your car at a time will provide close to the best economy possible using only this method, but you'll be back and forth to the petrol station all the time, and probably cancel out all your good intentions in the process
On a related note: I'm reminded of my ex-girlfriend's confusion at how much fuel she was using whenever I came to see her (She lived quite far away, so I went to see her for a week at a time every now and again). As it turned out: she usually drove her 900cc Fiat with no-one else in the car. Me, being 6'5 and about 17 stone, caused her to get about 50 miles less on a full tank of fuel.
So it can have quite a big difference, but the difference is a lot more noticeable in low powered, lightweight cars
Good thinking, batman!