These are relatively minor characters spanning many books.
But a husband-wife duo, two of my favorite fantasy authors, David and Leigh Eddings, had at least a couple of super-sized characters in their epic saga which spanned two five-book series and two stand-alone books.
The Belgariad, which was comprised of five books:
Pawn of Prophecy, Queen of Sorcery, Magician's Gambit, Castle of Wizardry, Enchanter's Endgame
The Malloreon, comprised of five books:
Guardians of the West, King of the Murgos, Demon Lord of Karanda, Sorceress of Darshiva, Seeress of Kell
And then two stand-alones: Belgarath the Sorceror and Polgara the Sorceress
These two characters of which I'm thinking weren't strictly main characters in the story, and now that I think about it, I don't remember either of them showing up in the two stand-alone books, mainly because those two were focused heavily on the lives of two of the main characters prior to the start of the main story.
But they appear throughout the series, and a good bit of thought was given to their characterization.
Anyway... the two super-sized characters.
One was a king, named Rhodek. He was portrayed as a thoughtful leader and a brilliant and skilled tactician. He was described as immensely fat with an enormous appetite, with many references to his weight and his vast belly. Deeply loved and respected by many of the other leaders and characters that played a big part in the story.
The other was a thief named Droblek. He was actually the leader of the Thieves Guild in one of the countries in the story. He was also characterized as a cunning leader, and even though he was a thief, he was considered a very reliable ally to the main cast. Also described as very fat.
And now that I've been recalling the various characters they created, I remember one of the main antagonists in another series of theirs.
The Elenium, comprised of three books:
The Diamond Throne, The Ruby Knight, The Sapphire Rose
This character was named Otha, and he became prodigiously fat over the unnaturally long course of his life. He was initially a simple shepard, who had stumbled upon the idol of an evil god to whom he basically sold his soul.
Being granted nearly-unlimited supernatural and political power, he fell whole-heartedly into his hedonistic urges, drinking and eating to such an extent that he became immensely bloated, like an enormous grub. His arms and legs atrophied to the point of being useless as he was carried in a large palanquin everywhere he went.