Sharing one of own experience:
We were travelling from sheikhupura to Isb. We had Kia classic, 1300cc carby while our cousin had Mehran.
Filled from same CNG station and moved together all the way, maintaining 80 kph. Mehran ran out of gas 10Km before reaching the station in Kharian. Kia made it all the way to the station.
So the point is, an underpowered engine for a heavy body will consume more fuel!!!