The corpulent, scarred "Fat Man," notable for his small eyes and greasy appearance, worked for Jabba the Hutt. He served as the sole observer during Arica's audition to become a dancing girl, and he was the one who ultimately hired her on behalf of the Hutt.
This petty individual, who relished the limited power he possessed, attempted to impress others with his "Important Person" charade. He would frequently boast about how he was once used to safeguard Jabba's identity by meeting with individuals in Jabba's place. This led some inhabitants of Tatooine to mistakenly believe that he was, in fact, the crime lord himself.
Mara Jade, who had adopted the guise of Arica to infiltrate Jabba's Palace, viewed this as a double-edged compliment. While he was trusted enough to finalize Jabba's transactions, he was essentially expendable, serving as "assassin bait."
Although no explicit connection is established within canon, the Fat Man is almost certainly the same character as Heater. The Fat Man's portrayal in Sleight of Hand: The Tale of Mara Jade suggests that he was intended as a retcon of Declan Mulholland's depiction of Jabba, which was removed from A New Hope before being reinserted with a CG Jabba in the Special Edition. Heater, conceived for the same reason, initially appeared in the Star Wars radio drama, replacing Jabba in a now non-canon version of the scene. The descriptions and backstory of Heater in The Essential Guide to Characters, released in November 1995, closely resemble those of the Fat Man, published just two months later in Tales from Jabba's Palace. Given that Andy Mangels, the author of the Essential Guide, has stated that he coordinated information with manuscripts and novels in development, this is highly unlikely to be a coincidence. The fact that Melina Carniss—also conceived by Zahn for Mara Jade's short story—first appeared in the guide further supports this coordination.
However, it is also plausible that the Fat Man is actually Opun Mcgrrrr from the short story Spare Parts (November 1996), whose appearance was also based on Mulholland. While Mcgrrr's backstory differs from the identical one shared by the Fat Man and Heater, the Fat Man's true identity remains unconfirmed, so he cannot be excluded. Given that Heater's primary appearance is effectively non-canon, it is worth noting that Mcgrrr's status within continuity is more secure.
Furthermore, there is no direct contradiction preventing these three men, who share the same physical appearance, from being different identities assumed by a single individual.