Why did Chouji get fat when he received Naruto and Kurama's Chakra?
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.