Why did Chouji get fat when he received Naruto and Kurama's Chakra? Why did Chouji get fat from receiving Naruto and Kurama's Chakra? Chouji didn't activate any technique for it to happen, actually he was surprised. 

Do Akimichi bodies just naturally convert chakra into fat?
According to Naruto Wiki, the Akimichi clan:


  Members of the clan possess great physical strength and are able to
  quickly convert calories into chakra


So, they have the ability to convert calories into chakra, apparently they can do the opposite too, getting chakra and quickly convert into calories.
Maybe this one never happened to Chouji before, because he never received this amount of chackra, so he was surprised.