Reason for the output capacitance in boost converter being higher than that in buck converter
In boost convertors, for part of the cycle, all the load current comes from the capacitor. 

Buck converters can deliver current continuously through the inductor to the load (alternately from the voltage source and from ground), so only a fraction of the load current ever comes from the capacitor (in continuous current mode).

Therefore, for the same ripple voltage, the capacitor can be that fraction of the size of that in the boost convertor.

(This is not the whole answer; a more accurate answer would take the different duty cycles for different voltage conversion ratios into account too)