Nice comparative chart by nature guide Kumasuke, showing the size differences between heron/egret species found commonly in Japan:
Daisagi: great white heron or great egret. Note how the blue-green gape going past the eye
Chûsagi: intermediate egret or yellow-billed egret. Note the yellow gape stopping under the eye
Kosagi: little egret. Note the yellow feet + the fluffy crown
If I remember well, most egret/heron species slightly change coloring and/or grow fluffy feathers during mating season. Those feathers were once used for kanzashi hairpins, a remnant are the katsuyama kanzashi worn by maiko for Gion matsuri (see the butterflies faint "antennas"?):
Nowadays egret are protected and feathers still in use are from ancient stocks.
Egret species are often virtually impossible to tell apart in Japanese patterns. On this blog, I simply tag any of those birds under "sagi" :)