We eat ONE unit of something—one plate, one bottle, one portion—regardless of how big that unit actually is. The same person eats 200 calories from a small plate or 500 from a large one. The "unit" is what matters.
Same portion of lasagna on different sized plates:
Both diners eat "one plate" and feel equally satisfied—
but one consumed 67% more calories!
Leaving food on a plate feels wasteful. Finishing "one thing" feels right. The unit defines the goal.
We're terrible at estimating volume. A plate that's 20% larger seems "about the same" but holds 40% more.
The package, plate, or serving defines "normal." We trust these cues over internal hunger signals.
"One sandwich" requires no calculation. Portion math is effortful; units are simple.
Portion sizes have exploded: A 1950s soda was 7oz. Today's "small" is 12oz, medium 21oz, large 32oz. But people still drink "one soda." Bagels grew from 3" to 6" diameter (4x the calories). Restaurant plates grew from 9" to 12". Each increment seems small, but cumulative impact is massive—Americans now consume 570 more daily calories than in 1977.
9" plates instead of 12". Same visual "fullness," 40% fewer calories.
Hold less than short wide ones, but look the same size. Pour 30% less.
Divide family bags into individual portions. Each bag = one unit.
More expensive, but "one bag" is now 100 cal instead of 400.
The second plate is a new unit. Make it a conscious decision.
Two people, one entrée = half the unit for each.