"But why on earth does it matter if you achieve this with two large boxes or two smaller boxes and one larger one?"
I actually think it does matter quite a bit?
I am sure Hugh or another has covered this before but a single subwoofer is always a compromise. The general advice is I seem to remember that getting what LF you can from the monitors is better and spend the money there rather than a sub if possible and the smaller the room the less useful a sub becomes.
There is, as I understand things a very strong case for TWO subs to augment smaller speakers and these should be placed as near to them as possible.
A refresher article perhaps SoS?