Add a third item for comparison - a 1 kg bottle of water. That weighs more than either the Snickers bar or cabbage, but it's clear why you won't gain any weight from drinking the water (assuming you weren't dehydrated and you've had a chance to piss it out) - there's nothing in the water your body can use for energy now or convert to something it can store and use for energy later. The water just passes through you.

The bulk of the cabbage passes through you too because it's over 90% water. Most of the rest is insoluble fiber that just passes through our digestive system unused. Take away the water and the insoluble fiber, and you're left with a tiny bit of actual usable nutrients, and only some of that is potentially storable by the body.

A Snickers bar is about 6% water, and the bulk of the remaining 94% is calorie dense fat and carbohydrates (with a bit of protein). The body uses some of that for immediate energy and stores almost all of the rest, resulting in weight gain.

This is the general reason why vegetables seldom lead to weight gain - they're mostly water and whatever is left isn't terribly calorie dense(ignoring starchy veggies like potatoes and fatty veggies like avocado for the moment). Doesn't mean you can't gain weight eating only veggies, but it's much harder to do compared to eating more calorically-dense foods.