This argument isn't even anywhere close to making any sense. The amount of waste that's left after food is digested doesn't indicate anything about the nutritional values of the food you've consumed. The reason there's more waste after eating veggies is because veggies generally contain high levels of fiber and the human body cannot digest fiber.

(Aside: this doesn't mean fiber isn't good or useful for you, consuming fiber is a key component in keeping your gut fit and healthy because your gut has to do work to deal with the fiber, similar to lifting weights for your other muscles.)