The boot of the Mazda 3 III is only 11 % bigger than the Kia Spectra I Hatchback. However, the difference is so little that it would be right to say that both vehicles have more of less the same trunk space.
Trunk capacity with seats down
Distance between two sides of a vehicle affects interior space for travelers as well as stability during driving. The width of the Mazda 3 III Hatchback is larger only by 331 mm in comparison to the Kia Spectra I Hatchback 1.6 i 107HP and is not really relevant. The medium for the „small family cars” class amounts to 1,703 mm and is by 353 millimeters larger than the width of the Mazda 3 III Hatchback.
In the aspect of fuel tank size it is the Mazda 3 III which can carry 4 percent more fuel compared with the Kia Spectra I Hatchback 1.6 i 107HP. The medium fuel tank size in the C segment, from which the Mazda 3 III comes from, amounts to 52 liters, whereas in lower middle segment (represented by the Kia Spectra I Hatchback 1.6 i 107HP) it amounts to 52 liters.
Mazda 3 III Hatchback 1.5 SKYACTIV-G 100HP manual has 149 mm longer wheelbase in comparison to the Kia Spectra I Hatchback. A longer wheelbase gives more stable and comfortable ride, especially when driving fast. It also provides more storage space. But, vehicles having a longer wheelbase will not be as maneuverable as those with a shorter length between its axles.
The higher a vehicle is, the better is the comfort of getting in to it and greater possibility of loading big amount of stuff to it. The difference of 25 millimeters in favor of the Mazda 3 III Hatchback is small and does not matter much.