More CC doesn't mean less mileage, a 1.8 Altis gives better mileage than 1.3 due to better power to weight ratio. Even if the 1.8 is manual, it will be at a lower rev range when driving at constant highway speed as compared to a 1.3, which will resultantly give better mileage.
A 660cc engine will have to work harder to maintain higher speeds, driving slightly slower might increase your mileage too.
Another factor, you mentioned that you drive with the AC off on highways. Do you keep your windows open for air? If that is the case, your car will have more drag (will be less aerodynamic) it will have to work harder to maintain it's speed, this will also affect your mileage. If you are driving above 80, it's better to switch on the AC and keep your windows up, rather than keeping the windows down with the AC off. Keeping the windows up and AC off will give the best mileage but the torture won't be worth it.
Also, are you using the correct viscosity of engine oil?
