The Bank of Japan's reduction of the amount it invests in the Nikkei 225 stock average is leaving the blue-chip gauge lagging behind the broader Topix by the widest margin in nearly two years.

Following criticism that its exchange-traded funds purchases are distorting the stock market, the central bank said last September it would reduce its Nikkei 225 buying in favor of the Topix, which covers all stocks in the Tokyo Stock Exchange's first section.

While a 26 percent slide this year by Uniqlo clothing stores operator Fast Retailing Co. — which has the heaviest weighting in the price-weighted smaller gauge — has contributed to its underperformance, falling out of favor with the BOJ is having more of an impact, according to analysts.