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My first post.
I agree the weight will have very little effect on the mileage, especially highway mileage. Weight in general has very little effect on mileage when a constant speed is maintained. If I'm thinking about it correctly (thought experimentation), weight will only have an effect on gas mileage at a constant velocity by increasing the rolling resistance. Therefore, shaving 50 lbs on the wheel+tire assemblies (that's a huge shave) will have an effectively immeasurable effect on constant-velocity fuel economy.
The factors effecting rolling resistance are not just the width, tread pattern and rubber compound, but also the age and condition of the tire (rubber hardens over time = more mpg), and remaining tread depth (less tread depth = more mpg).
Going from an old tire to a new tire of the exact same brand, model, size, etc, can measurably decrease fuel economy. This often leads people to the inaccurate conclusion that the new type of tires they've chosen are significantly less efficient than their old ones.
Furthermore, differences in sidewall height can change the tire pressure needed for the ideal contact patch. If both the 15" and 17" setups are running the same pressure, this can actually be an inconsistency in the testing.
In short, weight is not the reason for the change, and there are too many other unknown variables to make a solid conclusion about economy based on the information provided.
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