I actually have studied Japanese for a while and have no clue what you are describing as extremely long.
In hiragana you have 48 letters for sounds which in most cases are a consonant + vowel. Hence Japanese written in Hiragana or Katakana is shorter than it would be if it was written with a western alphabet. Japanese is probably not the language with shortest words, but neither are they exceptionally long.
Romaji is not in use for the most part in Japanese. Romaji would of course be much longer but I was talking about a sentence written out in only kana vs a sentence written out normally with a mixture of kanji and kana.
In hiragana you have 48 letters for sounds which in most cases are a consonant + vowel. Hence Japanese written in Hiragana or Katakana is shorter than it would be if it was written with a western alphabet. Japanese is probably not the language with shortest words, but neither are they exceptionally long.