But satiety is defined as the state of being full. The satiety index should be based on volume not calories. Otherwise it’s not just unintuitive, it’s incorrect.
You don’t have to consume a certain number of calories to be full. You just need a large enough volume. Competitive eaters eat a huge amount of lettuce to feel “full” but it has almost no calories.
You can eat 500 calories of butter and not feel even slightly full, but 500 calories of raw potato would have you full to bursting.