Climate has a big influence on the growth. The warmer Japanese climate, high air humidity and long growing seasons make the Japanese bonsai grow with great speed and far quicker than us using the northern hemisphere as base.
The shorter growing season here do develop trees in a lower tempo than in Japan. At a talk with Tomohiro Masumi from the Koju-en shohin nursery in Kyoto, we discussed the advantage and disadvantages of this is, and like Tomohiro pointed out, northern Europe countries (like Denmark and the UK e.g.) will be able to keep shohin trees in size and form longer than the Japanese. In Japan the problem of the expanding trunks and heavy growth shows the difficulty of keeping the trees in shohin size for a long time.
After 40 years with shohin in Japan, many of the trees are not possible to keep in the size under 20 cm anymore, and therefore the Chohin category is developed for the purpose of finding a room for the now former shohin outgrown. I will therefore like to point out that it is a small misunderstanding in the west to name medium sized trees Chuhin at exhibitions, because these trees have not been shohin at an earlier stage. The Chuhin category is for shohin outgrown their size, and now exhibited as this.
Despite slow growth and longer time before a tree is developed, we can enjoy having our trees the right size for a longer time. Some advantages to live in a colder area luckily 🙂