When blood pressure cuff is too small, the blood pressure reading will be inaccurate, in the direction of being too high. (How big is your bicep?) The reading you obtained of 180/60 is odd, because your systolic BP of 180 is VERY high, and your diastolic BP of 60 is rather low, especially when we recall that the reading is probably inaccurately high due to your arm being bigger than 15 inches. Thus, I suspect the machine may have been malfunctioning. More commonly, when SBP is very high, the DBP will also be at least a little bit high... not always, but very often. At any rate, you definitely ought to recheck it at least a few times. No one should put too much stock in a single reading, as BP fluctuates. Keep in mind that it actually helps to be in a calm, relaxed state of mind, and maybe sit down for 5-10 minutes before the reading so your body is in a resting state.
Good luck!