Ah, found it: this is a much better link for the story: http://english.peopledaily.com.cn/90786/8044143.html

"During the experiment, four kinds of edible vegetables, namely lettuce, leaf lettuce, gynura bicolor and sonchus oleraceus, were planted in a 36-square-meter area to absorb the carbon dioxide exhaled by the two astronauts and provide them with oxygen and 30-50 grams of fresh vegetables per meal per person."

Google tells me you need 1 ton of soil to cover 30m^2 to a depth of 25cm. So now I'm interested to know if/how they're optimising for mass efficiency.