The project was conceived by Dutch artist Florentijn Hofman. Each country or city that chooses to display a Rubber Duck builds one from scratch and they therefore vary in size. The largest one so far measured 26 × 20 × 32 metres and weighed in at 600kg. Seoul's duck measured in at 16.5 × 19.8 × 16.5 metres. I'm not sure if it weighs the same when it was seriously ill.
From what I can tell the duck was built by Lotte in order to promote their newly opened World Mall. Clues were:
Image taken with a Samsung Galaxy S4, edited with Snapseed.