The higher-capacity drives ought to be faster even at the same rotational speed because more data passes under the heads each millisecond.

Moving from say, a 120 GB drive up to a 240GB drive ought to give 2X the data rate if the extra capacity was achieved by extra platter(s) in the drive, or 40% higher rate if it was a simple density increase, such as from the perpendicular technology now showing up.

Obviously, going up to 7200 from 4200 is also a big — approx 70% — gain, but there are other advances at play, too.