Actually, required size of the transformer drops with the frequency. Mains switching power supplies still use regular transformer for isolation which run on comparatively high frequency and thus can be very small.

50/60Hz mains is motivated by compromise between visible flicker from lightbulbs and required frequency for practical synchronous motors with useful rotational speeds (some electrified rail systems use lower AC frequencies because they are better match for motors). Also, significantly higher frequencies will cause even relatively short mains runs to behave like a (RF) transmission line and also cause more significant skin effect.