Baron Harkonnen in Dune is the ur-example of this trope. Described as so humongously gluttonous that he requires anti-gravity generators in his clothing just to hold himself up. Mentioned as being "baby-fat", so get out a picture of a fat baby, and then imagine a full-grown man having that percentage of body fat! Eww.
In the somewhat contested prequel novels, the Baron's obesity is actually a consequence of a disease he contracted when he violently coupled with Mother Gaius Mohiam to conceive Jessica as per his agreement with the Bene Gesserit. Mohiam inflicted the disease on him as payback for his brutality during the act. When he learned that it was incurable and untreatable, the Baron decided to invoke this trope to let his enemies think that his obesity was a sign of indulgence and excessive wealth rather than a sign of weakness.
A few villains in Redwall: Clogg, Bowfleg, Agarnu and Bladd. Clogg is known to eat/drink whatever may be by his side when he wakes up.
The arachnid Shelob in The Two Towers She wants nothing more than to consume everything she can, orc, man, elf, or hobbit.
Ungoliant, Shelob's mother from The Silmarillion, also wanted to consume everyone. In the world. Even Morgoth, the Big Bad of those days, was scared of her. And for a good reason — Ungoliant ate all the artificial gems Morgoth had stolen, after having drunk all the Light of the Trees. *burp*. She would have eaten the Silmarils and Morgoth if he had not summoned some Balrogs to drive her off. Eventually she went North and ate herself. Ungoliant's probably the closest the LOTR universe has to a genuine Eldritch Abomination, and the codifyingAnimalistic Abomination.
Swelter, the evil and overweight chef from the Gormenghast novels.
Sydney Greenstreet's portrayal of Wilkie Collins' classic character Count Fosco in the film of The Woman in White was in keeping with his portrayal in the book; more recent portrayals, sad to say, have not been.
Each of the villains in the Keys to the Kingdom series represents a deadly sin, with Drowned Wednesday representing gluttony. This trope is averted, however, when it turns out that she is no longer a villain.
In Eoin Colfer's The Supernaturalist, Mayor Ray Shine is suggested to be huge, and is most villainous.
In Michael Chrichton's Timeline, the protagonists are surprised to see that this trope is inverted. The "good" king is the one with Jabba Table Manners, stuffing huge gobs of meet into his mouth constantly, while the "evil" king has perfect manners.
The appropriately-named Casper Gutman from The Maltese Falcon. (Dashiell Hammett, in his earlier career at a detective agency, was involved in the Fatty Arbuckle case, and some thing his fondness for fat villains stems from that.)
In the Rainbow Magic series, while most of the goblins aren't fat, they're very greedy when it comes to food. One of the conflicts in the Sugar and Spice series came from them eating Jack Frost's Candy Castle.