Energy density. Chocolate is very energy dense, so you can fit a lot of energy in a small bar. You can get your entire day’s energy needs from that bar.
To get the same energy from lettuce, you’d need to eat 10 kg.
Huel is saying that junk food fills your “gas tank” fast, but it doesn’t satiate your feelings of hunger.
It’s to do with the nutrients that you are eating and their release time. This is best exemplified comparitivly between wholegrain pasta and dextrose. Dextrose will be synthesised quicker so you get energy quicker but it lasts for a shorter period of time. Wholegrain pasta co times complex carbohydrates which need to be synthesised for a longer period of time and release energy over a longer period of time. This therefore leaves you feeling fuller for longer but with less the amount of calories.
Calories are just units of energy - they’re nothing to do with how filling something is. A bottle of wine can have 650 calories, but that won’t fill you up like a 650 calorie meal.