I know @haejin said that, but that's not the understanding I have, nor what http://stockcharts.com/school/doku.php?id=chart_school:chart_analysis:chart_patterns:head_and_shoulders_top_reversal says either.

Volume: As the Head and Shoulders pattern unfolds, volume plays an important role in confirmation. Volume can be measured as an indicator (OBV, Chaikin Money Flow) or simply by analyzing volume levels. Ideally, but not always, volume during the advance of the left shoulder should be higher than during the advance of the head. This decrease in volume and the new high of the head, together, serve as a warning sign. The next warning sign comes when volume increases on the decline from the peak of the head, then decreases during the advance of the right shoulder. Final confirmation comes when volume further increases during the decline of the right shoulder.

The head has lower volume then each of the shoulders.