I don't remember exactly why, but Sheldon (peace be upon him) said you can't use smaller than 16T on the nexus.
Because the shifting mechanism is integrated instead of using a clickbox. Therefore you need a dished sprocket. If you use a flat one, then the chain might rub against the hub. The low gears was the main reason I sold my Curve SL. Highest chainring I have seen in 130 BCD was 70T (custom made), and the highest one I found from an online retailer was 60T.