Why 400 Hz in Naval and Air applications?

At 400Hz all electrical equipment like alternators, Transformers and electric motors become smaller to supply the same power than 50 or 60Hz equipment, they are using Bosh 90 amp auto alternators same size as BMW and Mercedes with some internal modifications that is capable of up to 8.4 Kw and 70 Volt 3 phase that is then inverted to 220 Volt with a transformer