As a Japanese native who's fluent in English, I disagree with this. I regularly translate English (mostly technical) texts to Japanese, and the translation I get is roughly the same size as the original. Its density is certainly not "very low".
The reason that you might think that Japanese is more "bloated" is probably because the form of the language you hear most - in TV news and speeches - are in the "polite" form. In that case, they mean to be verbose. But in the casual form, the same thing can be said much shorter.
Also, keep in mind that Japanese has a very different phonetic system. Their phonetic intuition is mostly based on "mora" and many find the notion of syllable confusing and inconsistent. I don't think these two are very comparable (at least mentally).
It's funny that you mentioned that a Japanese newspaper looks more "crowded" than English one. Japanese people would think the exact opposite. The "information density" is really in the eye of the beholder.
The methods this paper employed may certainly be biased in a way that makes Japanese appear less information-dense than other languages.
As for the _written_ language's density, that's entirely different! You say that as written, translations come out to be about the same in Japanese and English, I assume this corresponds to roughly the same number of characters as well? If so, if we can believe the authors of the paper above that Japanese has less information-per-syllable, then perhaps Japanese writing's syllabic/ideographic writing system accounts for this handily such that the information density of _written_ Japanese is about the same as written English.
For what it's worth, I think you were agreeing with me actually:
> Take a look at the front page of a Japanese newspaper and show it to an English speaker, they will think it's "crowded", but I bet if you translated it to English you'd see something that resembles a typical English paper.
What I meant here was that in terms of _written_ information density there are clearly significant gains made thanks to the writing system.
It's complicated, but perhaps a good comparison is newspaper headlines in the West: headings and phrases etc are compact without ever being slangy.
For example, a random ad on the Yahoo Japan homepage proclaims 予約来場いただくと進呈QUOカード５０００円分, which means "get a 5000 yen gift card if you come visit", but uses literary words like 来場 (arrival/visit) and 進呈 (gift) that would rarely be used in spoken Japanese, and even the polite いただく (to humbly receive) is in the short "dictionary" form instead of being conjugated out the way it would be in speech.
Depends on the type of websites. Most corporate websites are polite. In a message board like 2-chan, (unsurprisingly) not at all. Actually, some of the most vulgar Japanese that most people would never speak in real life can be only found on the Internet.
The reason that you might think that Japanese is more "bloated" is probably because the form of the language you hear most - in TV news and speeches - are in the "polite" form. In that case, they mean to be verbose. But in the casual form, the same thing can be said much shorter.
Also, keep in mind that Japanese has a very different phonetic system. Their phonetic intuition is mostly based on "mora" and many find the notion of syllable confusing and inconsistent. I don't think these two are very comparable (at least mentally).
It's funny that you mentioned that a Japanese newspaper looks more "crowded" than English one. Japanese people would think the exact opposite. The "information density" is really in the eye of the beholder.