There are lots of different types of Japanese knives. Those that are thinner at the spine are called lasers and those that are thicker are called a workhorse.
Heavier knives are best for cutting carrots, celery, and other low profile vegtables. Lasers are better suited for large onions, sweet potatoes, winter squash etc. There are also many medium weight knives for all around use.
All good Japanese knives are thin behind the edge, or have a thin grind to the edge regardless of the thickness of the spine.
Japanese knives also have many different types of grinds. Convex, s grinds, single bevel, double bevel, wide bevel, symmetrical, asymmetrical, and so on. Profiles also vary from maker to maker and manufacturer.
The Japanese have many different types of specialty knives, all require different thicknesses and profiles and grinds. For the most part, in America, we use a knife and fork to cut our own food, in Japan they use chop sticks so everything is neatly cut up for you.
Overall Japanese knives are hard and thin. Not all Japanese homes have good knives, in America, most homes have very poor cutlery.
Hoss