This Cyberman was a lietentant of the Cyberleader and was therefore allowed to eat everything he wanted until he got fat. The other Cybermen used to tease him about being fat, because to a Cyberman anything different is wrong. To prove that bigger is better, the Cyber Commander attacked a colony of humans single-handed, and killed every last human in the whole colony, prompting the Cybermen to instantly think he was lying. To prove he did carry out the attack, he invited all the Cybermen from his unit to visit the planet. They were impressed, but laughed again when they realised the colony had no defences to speak of.
Outraged, the Fat Cyber Commander had had enough. He stormed off and activated a defense system built in on the ship, and detonated the Cybership with all the Cybermen still on board, obliterating them all. He danced around in his victory, but soon realised that he was alone on the planet with no means of escape.