Tyrion is often described as a "dwarf" or "imp" due to his short stature, but he is not actually fat. He is known for his sharp wit, intelligence, and cunning, rather than his physique.
It's possible you're remembering Tyrion's character differently, or perhaps there is another show with a similar character that you're thinking of. Can you provide any other details about the show or the character to help me figure out what you're looking for?