content: I may be wrong(not a miner) but I believe it is due to diamonds not being present in the crust normally, only where there has been a magma flow that has brought them to the surface.  This means that there is basically a column of rock that forced it's way up from beneath the crust that contains the diamonds and the rest of the area around it is essentially empty of them, so mining anything more than just specifically that column of rock is a waste of time for them.  My assumption is that the column is significantly smaller than the opening of the mine, but they have to widen it that far to maintain stability of the hole, but again, I'm not an expert and hopefully someone with more knowledge can chime in here.