It is currently drilling a 17.5-inch diameter hole -- smaller than the 28 inches required to rescue the miners -- in an effort to realign after veering off course over the weekend.
Drilling a 2 or 3 foot hole can be done with normal equipment, while drilling a ten, fifty, or one hundred foot diameter vertical hole would require an entirely new invention.
With skyscrapers, the deep foundation systems overcome this issue by drilling or hammering in a series of relatively small foundation piers (3ft or 1m in diameter or so).
Inside the riser pipe is a space about half a meter in diameter, large enough to hold the spinning drill pipe and bit and to allow the return flow of drilling fluid.