>Being much more calorie-dense, the Snickers bar would naturally result in greater weight gain

The weight you gain by eating isn't based on the weight of the food, but the calorie content. It seems like you get this concept.

The four macro nutrients in food are water, fat, carbs, and protein. However, water doesn't provide any calories for you to use. If you remove all the water from both foods, the Snickers bar is actually heavier than the cabbage.