To Thine Old Self Be True - S7-E20
Corrected entry: In earlier episodes Niles has talked about how Maris' food issues go back to her childhood, for example that she had to be taken to hospital to be force-fed as a child. But in this episodes, he claims that she was always chubby as a child.
Correction: "Childhood" can be any year before a person becomes a legal adult. It's quite possible that Maris was chubby before age twelve or so, then she developed the issues in her teens and had to be force-fed. Indeed, eating disorders and food issues often start in the teenage years.