P.S. It would be helpful if you told the source. Where did you read these sentences?

It’s a series about diets

It is odd, and slightly old-fashioned. Rarely, "To have made a meal" can mean "to have eaten." The "as a meal" part means that what you have eaten is your entire meal. For example, "In the morning, I found some cold leftover pizza in the refrigerator, and I made a meal of it," that the speaker ate the pizza as his breakfast. "You have made a poor meal" means "you didn't eat enough."