I believe Kurama has at least as much chakra now as he did back then, otherwise he simply would not be as strong as he is... remember, chakra regenerates.
Although it might be plausible to say that he is pure Yang chakra now, and therefore takes on a more Yang-esque form. Yang is the chakra of life/light... perhaps that's why when Naruto goes full Bijuu mode Kurama is formed by pure Yang energy. Philosophically speaking, without both Yin AND Yang energies, matter cannot exist. Things like fire would be considered pure Yang energy, if I'm not mistaken, which may be why Kurama does not form in fully solid matter, and instead is transparent; he is pure Yang energy now.
This is just my theory, but if Kishi had any REAL reason to make Naruto's BM look the way it does, that would be the explanation. If not, then in my opinion there is no reason, and it is just to make Naruto look "special" so as to differentiate between him and a "normal" jinchuuriki (though Naruto's Kyuubi Mode is pure Yang as well... so in my opinion that speaks volumes about why I believe my theory is correct), or just to look more like So6P as Roby said.