Look at the following chart. Plant based foods are all 500 Calories a pound or less. Animal based foods are all 1000 calories per pound or greater. So those who eat more plant based foods are lighter while those who eat more animal based foods are heavier.
Plant fats (oil) and animal fat (saturated) are both at the highest levels of caloric content so avoid both to stay or get slim.
For those who are interested, wine fits in at a cool 400 calories per pound. (3 x 5 ounce drinks)