You can’t eat 1000 heads of cabbage in one sitting. Your stomach would rupture and you would die. You have to poop it out first.

Cabbage is mostly water and fiber. Your body can’t digest fiber, so you don’t get anything out of it (other than it giving your intestines a good cleaning scrape on its way out). You don’t get fat from cabbage, because virtually 100% of it just goes right into the toilet.

A Snickers bar has lots of delicious fats and sugars in it. Your body can use that. If you eat more calories than you use with exercise, then you gain fat. Since your body thinks you are a caveman who needs to store fat to survive times of drought and famine, instead of a lazy fatass with a thousand Snickers bars, it stores as much of that energy as possible. It converts it into fat. A lot of it will still come out as poop, but your body can retain much more of the Snickers bar than it can the cabbage.