Many here and otherwise mistakenly equate engine capacity with mileage. That is only valid up to certain extent.
The lower engine capacity means that while bearing similar weight (passenger, luggage, etc) the lower capacity engine will have to work harder to maintain speed and in start/stop conditions and thus any high fuel mileage obtained under specific favorable conditions will vaporize. On the other hand, a higher CC engine with similar loading will have to work less hard thereby giving better mileage.
Driving style/aggressiveness and keeping RPMs in the sweet spot also matter a lot.
Did you change oils and other fluids after your purchase?