Do you have any numbers to back that up? A 747 burns 10-11 tons of fuel per hour and carries about 360 passengers. For a 12 hour flight, that is about 350kg of fuel per passenger. Starship's tanks hold approximately 250 tons of fuel and 950 tons of liquid oxygen. In order to match the per passenger fuel efficiency of a 747 they would need to carry about 700 passengers. Starship has about 20% more internal volume as a 747, so it seems likely that they would be able to fit at least the same number of passengers. That puts them at about 50% of the fuel efficiency of a 747.
In addition, methane combustion produces 2.75kg of CO2 per kg of fuel, while kerosene produces 3kg CO2 per kg of fuel. If they are able to fit 20% more passengers than a 747 due to the larger internal volume then they would be within about 66% efficiency on a CO2 emissions per passenger basis.