Think of the circuit size as "maximum capacity". More capacity doesn't hurt, too little can be a problem (just like my paycheck). You can plug a tiny incandesant (eg. 40W) light bulb into the same circuit that will run a toaster (eg. 1500W), each will run without problem.
Taken to extremes, more capacity can be a problem. There is a bit more cost due to cost of wire. At least in North America, there can be different shape plugs and outlets for different capacities that may limit what can be plugged into the outlet. If a short should develop, the larger wire and breaker means there could be a lot of power (creates more heat and shock hazard) flowing in places you don't want it (including through you).
In general I agree with the idea if having some surplus capacity on every circuit in case you should ever buy something that needs a bit more push. Just don't go crazy....
On edit, Tony posted as I was still typing (slowly). Funny that we both chose the 40W bulb as an example.
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