I know, take my name for instance. Muyuu (夢遊) it means sleepwalking. In conventional Japanese it would be considered three syllables mu+yu+u. Yu+u are pronounced as a tone "un-switch" after the "yu" - this is why Japanese pronounce so fast, they also make use of a tonal (bi-tonal, tri-tonal depending on accent) system. If the following u was a different proper-syllable, the tone switch would happen (not necessarily a different word). Words are not separated by spaces, neither in speech nor writing. All this is completely "unofficial" in the sense that's not taught formally, like a lot of other things in Japanese. Kids already have this "embedded" when they first go to school.

I do natural language processing in Japanese and typically texts in Japanese say more in less space than English, by far. Both in spoken form and in written form. They say about as much as in Chinese, when both texts are written by native speakers. Now, you cannot compare texts 1-to-1 because when texts are translated, they are remarkably less information-dense. There is a whole lot lost in translation and "not-assumed" when the original writer didn't have a native Japanese context awareness. Reversely, when the text is translated to Japanese it will often be less information dense but not as much. Partly, because the translator will find a balance between not adding information (a basic translation principle) and not sounding unnatural, as this would also take away the attention from the reader. Both cannot be achieved to a high standard between languages as different as, for example, Japanese and English. In the tests I do, I can distinguish with remarkable accuracy when the text was originally written in Japanese or not, and they are all native texts and translations.

Japanese is not just a language, it's also a whole culture and civilisation to an extent most other languages are not, except maybe Korean (who are some sort of bizarro Japanese to be honest... just don't tell either them or the Japanese). Chinese language is shared by remarkably different cultures and ethnics, they don't have the same level of common context. European languages have nowhere as much context.

In Japanese, you'd have a conversation going like this:

A "So, should I leave the umbrella by the door or can I put it to dry open in the aisle?"
B "un" (colloquial Japanese "yes")

And, given features that are totally lost or implicit, they'd now exactly what they mean. They'd answer negative, double-negative, even triple-negative questions with monosyllables and know exactly what they meant without any confusion or hesitation.

For me, the benchmark is seeing how much is lost in translations and subtitles in films. Japanese ones subtitled in English lose a massive amount of detail even in the most common conversations. The converse is nowhere close as remarkable. They simply have a much more complex set of rules for social interaction. A lot of what they say is customary but also untranslatable. A lot of what they don't say goes completely unnoticed by a foreigner, because the omission of these customary expressions also carries a lot of meaning. In short, it's not possible to compare the expressiveness of such a different culture, and the way we are even trying to measure it just shows how much we're "not getting it".