Check the official figures to check how far you are on both cars from the urban consumption.
Also, the Civic may be on the same tax category but the emissions have a scale to range from X value to Y value. If the Civic is on the lower range for that tax band and the 6 on the highest, then it will consume more.
In general, because it is a bigger size car, it will consume more. I think the official figures should indicate that.
https://www.spritmonitor.de/en/overview/27-Mazda/251-6.html?fueltype=2&constyear_s=2010&constyear_e=2010&powerunit=2Over here says that the urban consumption is 10.39l/100km which is 27mpg in old money. So you are there. Don't worry, the car is still tuned to the previous owner. Gradually, it will start performing according to your driving style but it will take a while.
Also, if you rely on the trip computer, you would get more accurate figures if you do not reset the trip computer and let it accummulate lots of miles to make the averages closer to reality.
In practice, you could fuel up to the first click, reset the odometer counter and when you fuel up on the next time, write down the miles and the quantity you used. Reset the odometer and keep doing that to keep up your own records and compare with the trip computer. You can adjust the trip computer to give you more positive or negative figures if you find out that for the distances you measure, it gives you more or less mpg.