Calories are pretty much the standard metric and it makes sense since they're the energy unit. You burn the food and measure the heat it generates, boom: calories. Protein burns and makes heat, fat does, sugars/carbs do. It makes sense to measure calories because they're measurable across components using the same methods. Mass doesn't seem relevant at all and skews, like you mentioned, based on caloric (there they are again) density, water content, cell structure, etc. Someone could eat a truly massive amount of leafy greens and get less sustenance (Calories+nutrients) than a small steak or modest bowl of porridge would give.