Of course the impactor itself is Mars sized, but this only means that the majority of its mantle also went into the Earth. Only a fraction of the impactors mantle (~20%) formed the moon - so the Earths Core is only "overabundant" in iron regarding to the relatively small mass lost to the formation of the moon.

Look at this example. All mass units in multiples of a Moon mass (where Moon mass = 1% of an Earth mass, Mars mass = 10% of an Earth mass for simplicity). Most of the debris is picked up in later collisions (see the Canup-Papers), so it is neglected here.

Theia: 6 mantle, 4 core.
Earth: 60 mantle, 40 core
Moon: 1 mantle
Proto-Earth (before Theia impact) = 60-5 = 55 mantle, 40-4 = 36 core.

Core-mass-ratio of proto-Earth: 36/(55 + 36) = 39.56%
Core-mass-ratio of Earth today: 40/(40+60) = 40%

So the impact increased Earths core-mass-ratio only by 0.44%. As I said: it makes no difference.