The cabbage has a lot of fibre that will pass through you (although this is good for your gut), and water as well that you'll just pee out.

You don't gain the weight of what you eat anyway. You need to process and digest it, which uses energy, and your body uses energy to heat itself, do basic processes, keep your brain running, etc.

Per gram, each macro (fat, carb, protein) has a different calorie value. It's based on how easy it is for your body to break it down and use the components as energy. Calories are just energy.

The macros/foods might have other properties. Protein is also used for muscle building and repair.