It seems that records for the largest floating solar power stations are short lived.
In early 2016 the record was held by one near London. 23,000 panels are located on a reservoir managed by Thames Water. It produces 6.3 MW, which is used to power local water treatment plants providing clean drinking water for London. As a comparison to the Japanese plant, 6.3MW is the equivalent electricity consumption of 1,800 homes.
In May 2017, China switched on a floating solar power station capable of generating 40MW and powering 15,000 homes. The power plant is located on a former coal mine site in Huainan in central Anhui Province. The unused coal mines collapse and over time filled with rainwater, leaving a lake which has been largely unused to date.
Floating solar farms often take advantage of areas that would otherwise be unused and the water helps to cool the solar panels, thus increasing their efficiency.