I think there is a misunderstanding about cleavers here, the difference between most Japanese made cleaver and Chinese made cleaver is not their thickness or their belly, it lies in their grind. The CCK 1302/1303 and KF1912 that's popular among a lot of people is a style of cleaver called Sang Dao/桑刀, it is mostly used in Cantonese cuisine but gained popularity nation wide. The characters of Sang Daos are their thin grind and narrower rectangular shape with a flatter edge than most. Since the early Chinese immigrants to west has been overwhelmingly Cantonese and Honkongese, they bring their cuisine and their knives, it become the standard Chinese cleaver, the Dexter cleaver thats popular in North Americas is based on the Cantonese Shape.
In contrast, while there are Cantonese immigrants to Japan, Chinese cuisine regained popularity in Japan after WW 2 after many Japanese soldiers and settlers returned from Northern China, brought with them things like Gyoza. One of the people who reshaped Chuka Ryori (Japanese Chinese cuisine) is Chen kenmin and his son Chen Kenichi, both of them specialized in Sichuanese cuisine and use Sichuanese style cleaver subsequently.
Now back to the Japanese made cleavers, most of them are modeled after a more workhorse styled knife called Pian Dao/片刀, which is used in most regional Chinese cuisine, some have more belly some does not, some has rounded head or heel depends on the region. The popularity of Northern and Sichuanese Chinese cuisine made Pian Dao much more popular in professional kitchen, most of Japanese produced Chinese cleavers are made in this style. The main difference is that high end Japanese cleavers will usually have a convex grind at the edge like Sugimoto and Ginga, whereas traditional Chinese Cleavers will have a full V flat grind.
There is also a cleaver that's more ubitiqous in China, Zhan Qie Dao /斩切刀 or Wen Wu Dao/文武刀, those are truly all purpose cleaver with different angle at front and the back for veggies and small bones. However like most things all purpose they excel at neither.
Here's some pictures
Typical Sang Dao
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A sub variation of Sang Dao that's specialized for slicing beef
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Sichuanese Pian Dao by Deng Jia with some belly
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A Beijinese Zhan Qie Dao by Wang Ma Zi
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A Shanghaiese Pian Dao with rounded head
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Traditional Chinese grind
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