In this paper, we empirically examine the extent to which product downsizing occurred during the deflationary period in Japan, as well as the effects of product downsizing on prices and quantities sold. Using scanner data on prices and quantities for all products sold at about 200 supermarkets over the last 10 years, we find that about one third of product replacements were accompanied by a size/weight reduction. We also find that a 1‐percentage point larger size/weight reduction is associated with a 0.45‐percentage point larger price decline, resulting in an effective price increase. Finally, we show that the quantities sold decline with product downsizing, and that the responsiveness of the quantity sold to size/weight changes is almost the same as the price elasticity, indicating that consumers are as sensitive to size/weight changes as they are to price changes. Our results suggest that the Japanese consumer price index may be downwardly biased rather than upwardly biased.