Pigmon was sort of a spin-off monster from the preceding show “Ultra Q” in which the monster appeared as Garamon, so there seems to be no design of Pigmon drawn by Narita.
The point is that the Pigmon costume was used as it was extended in height because the actor, Minoru Takahashi, who played Garamon in “Ultra Q” Episode 13 “Garadama” and Episode 16 “Garamon Strikes Back” didn’t participate in “Ultraman.”
When you look at the position of Pigmon’s arms, you will be aware of the extension, and, as the result, Pigmon looks much thinner than Garamon.
At any rate, the Garamon costume was extended (it is uncertain who did the remodeling) so as to fit a child actor who was decided to act the monster instead of Takahashi.
Pigmon is described as it was played by an actor named Shuji Fujita first, but his whereabouts are not know after his appearance in “Ultraman,” and Reborn Pigmon was performed by another child actor who was to be known as Hiroshi Chiba in later years who played one of the leading characters in a popular detective drama (as an adult actor).
When seeing the transition from Garamon to Reborn Pigomon through Pigmon, it makes us realize the thorny parts all over the body were getting out of shape gradually and the equivalents of Reborn Pigmon look utterly different (isn’t there any pasta like them?) from those of Garamon probably because they were replaced by new ones.
HIroko Sakurai who played Akiko Fuji says in a book interview that she remembers the crew added red to Reborn Pibmon’s body by spraying the color on the set in the toy department and her glove got stained with the color when touching Reborn Pigmon as the paint was still wet.
(Regarding Reborn Pigmon’s voice, see this post)