This size ratio is needed in order for the resulting system to have sufficient angular momentum to match the current orbital configuration.Such an impact would have put enough material into orbit around Earth to have eventually accumulated to form the Moon.Surprisingly, the Apollo lunar samples carried an isotopic signature identical to Earth rocks, but different from other Solar system bodies.
Along the way, the Moon's rotation became tidally locked to Earth, so that one side of the Moon continually faces toward Earth.
Also, the Moon would have collided with and incorporated any small pre-existing satellites of Earth, which would have shared the Earth's composition, including isotopic abundances.
The Moon's relatively small iron core is explained by Theia's core accreting into that of Earth.
The lack of volatiles in the lunar samples is also explained in part by the energy of the collision.
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