Saenchai won the Lumpinee Championship title, which is widely considered the most prestigious title in Muay Thai, in four different weight divisions, while mostly fighting larger opponents. He is considered by many to be the best pound for pound Muay Thai fighter, and is regarded as one of the best fighters of all time. Saenchai would often give up weight to find worthy opponents in Thailand.
Check out some of the best compilation of him beating up bigger guys: