Using a blood pressure cuff that is too large can give you a falsely high blood pressure measurement.

Using a blood pressure cuff that is too large typically produces a falsely low blood pressure reading. When a cuff is too large (wide) for the arm, it requires less pressure to occlude the artery because the pressure is distributed over a larger surface area. This causes the Korotkoff sounds to disappear sooner, leading to an underestimated blood pressure. Conversely, a cuff that is too small (narrow) requires excessive inflation pressure to compress the artery, leading to an overestimation.