Using inductively coupled supplies, higher frequencies allow reduced core sizes. This requires more capable switching devices. At present that would probably mean IGBT's in your consumer supplies.
Capacitive coupling is capable of magic if the switching frequency can be made MUCH higher - maybe a factor of 1000. People are starting to deliver. As CPUs that run in the GHz range have become commonplace the ability to switch power at such frequencies at half decent prices has followed, albeit reluctantly.
Consider a 0.01 uF capacitor - somewhat higher than the gate capacitance of a rather annoyingly capacitive MOSFET.
Energy transfer for full charge discharge = 0.5 x C x V^2 x Frequency
At a switching rate of 1 Ghz and for say 5V swing this gives
E = 0.5 x C x V^2 x f = 0.5 x 10^-8 x 25 x 10^9
=~ 120 Watts.
You do not want to discharge or charge the cap by more than a small percentage per cycle and to work over mains to equipment voltage ratios you need many stages and ... all of which makes life hard.
BUT once you get to 10 Ghz that's 120 Watt with a 0.001 uF / 1 nF, and if you allow early stages 30V swing (you don't) then that's about 4 kW for a 1 nF or 400 Watts for a 10 pF or 40 Watts for a 1 pF. At 1 pF you start to be able to make some very nice low loss caps and "rather compact" and ... .
Give it 10 years and the 10 GH cap switch supplies will be competing with the 10 MHz+ inductive switchers for compactness. Maybe.
You can buy data couplers now that power their isolated portion by capacitive power transfer in the GHz range.