I've heard of large-leafed duckweed but I've never seen it in person or seen a photo of it before. Duckweed will grow to different sizes based on lighting and nutrients. For example, I put a handful in an indoor pond, and a handful to an outdoor pond. The duckweed indoors with less intense lighting grew larger and greener with longer roots. The duckweed outside in full sun gew very small, light green with shallow roots, but it multiplied MUCH faster than the one indoors.
I don't think duckweed's roots get very long, ever, so if it's got long roots (over 1/2), it's probably some other floater like frogbit or salvinia.