The species Canna glauca has large seeds. It apparently was one of the species used in producing the large-flowered hybrids. It may be why the large-flowered hybrids the produce seeds often have larger seeds than some of the species. The commonly grown species Canna compacta (aka Canna 'Robert Kemp' - tall, green-leaved with many small, red flowers) also has seeds about as large as those of Canna glauca.

To replicate a canna, plant a rhizome. To get a surprise...plant a seed (any size). I planted seeds from an ambassador giant red canna. The seeds produced: Miss Oklahoma(hot pink), Ellen Gallow (peach), and a lighter pale yellow looking like En Avant. Seeds often regress into the plants used to make a particular canna. A Botanist confirmed this to me and showed me a dozen different cannas grown from seeds of two plants.