Only X number of bitcoins are "allowed to be found" per time-period. EG, if you mine with a kH/s machine, you would find 1 per year (solo). If you mined with a MH/s machine, it would adjust until it only allowed 1 per year (solo). If you mined with a GH/s machine, it will adjust until it only found 1 per year... etc...
Which is why it's best for those making ASICs to use them while they can make a decent amount of coins, and only sell them when the difficulty is such that it takes a couple of months to recoup the price of one.