For translated works, does the word count de- or increase in Japanese?

Not entirely sure if this is the right sub for that question, but I don't know where else to ask.

Essentially, my friend and I read the same book (Project Hail Mary), but she said her Japanese version is split into 2 parts (2冊) (and there shouldn't be a sequel). Kanji should make it shorter, right?

In German, the translation is as a rule of thumb 1/3 thicker than the English version, mostly because the added inflections. I tend to read books in English for better comprehension and nicer reading flow.
How does this compare in Japanese?
Also, if you are not an English native, does your translated version change in size?