I do not know what reason this suggestions provide. But I do not know one reason to do so.
The sizing is important for Physics. 1 BU = 1 meter.
Anything else (except I forgot something) is “scale-free”.
Ok, not really.
There are parameters spread around the whole application (e.g. margins, radius, gravity, velocity, camera distances). If you scale your scene the default values might not match anymore. Which means you need to adjust them manually. Sometimes this is not possible.
Finally it will not be faster (at least I do not know why it should be).
In general it is a good idea to keep in mind the default cube is not a little box.
It is = 2m³.
That means multiple adult character can stand upright in it (next to each other not above each other ).
I hope this helps