Power switchers copper fill / region
There is no glaring downside to overfilling the input and output nodes of a SMPS (if you have room) but the switching node of the supply should not be egregiously* oversized. 

This node is switching very fast (500kHz - 2MHz are common in most ICs) and usually from ground up to your input voltage so it generates a lot of high frequency noise. It is essentially a capacitor and the larger you make the plane pour, the more capacitance it has and the more likely it is to cause problems by coupling into your ground plane or high speed sensitive trace. 

*On most designs I generally size it about 1.5x - 2x as big as it needs to be (and am very careful not to run any other traces under this node on any layer) for a couple of reasons listed below.


If I accidentally short the output the trace (hopefully) doesn't pop and the
circuit is useable/salvageable. 
This can also protect you from throwing out the PCB due to
    requirement creep if you need more output current than initially
    thought