One of my siblings is obese and another overweight. I've been the 'skinny one' (at healthy BW/BMI/whatever) all my life. I've long suspected that if they could watch every single thing I consume in a day's time—or alternatively, allowed me to portion every single thing of theirs—they'd be shocked by the sheer difference in volume from what they've come to expect as normal.

Calorie counting aside, many, many people become strongly adapted to excessive daily consumption, and they're completely unaware that it's their root issue. It's not so much that weighing can show people how much they should be eating, but rather how much they should not be eating.