I thought so, too. Until I was in Japan for business, where I saw some books that were translated into Japanese Kanji. The books were significantly larger in every case than the English versions.
The hiragana and katakana are syllabaries and don’t have the information density that you get with kanji.
I don’t imagine you’d get Japanese books that are more compact than their English counterparts because per sentence Japanese texts are comprised mostly of hiragana and katakana with a sprinkling of kanji.
Chinese on the other hand is all hanzi, and that does provide significant information density.
I used to have the lord of the rings in both English and Chinese on my bookshelf. The 3 volumes in Chinese where about the same thickness as a single volume in English (width and height were approximately the same) and this was with a much larger Chinese font required for legibility.
Kanji is part of the Japanese writing system, it's not the entire thing. "Translated to Japanese kanji" is like saying "translated to English consonants".
> Kanji characters were printed significantly larger than ASCII, so that accounted for a lot of it.
Japanese books also have a lot more whitespace and are generally smaller than western books. I disagree that the font is much larger -- the books I read have a font size that is practically identical to most western books (the difference is that CJK characters are more boxy and so they may appear larger).
> ASCII can have a pretty small font and still be perfectly legible - apparently not so with Kanji.
You can print kanji pretty small and have it still them be legible to native speakers (just look at the first video games that used kanji instead of just using kana -- the effective font size is tiny).