At first, Naruto couldn't fully transform into his tailed beast when using the full extent of Kurama's chakra. This was due to the hostility that had existed between Naruto and the fox. When he attempted to transform into his full Nine-Tails form, he instead transforms into a miniature, comical version of Kurama. It was also shown that Naruto cannot maintain this form for very long, as it drains him of its chakra significantly.
