pistachio principle
n. The tendency to eat less food given certain visual cues, particularly evidence of the amount of food consumed, such as pistachio shells; the tendency to eat less of a food that takes more time and effort to consume, such as nuts that must be shelled.
The point of the "pistachio principle" is not to encourage eating fewer nuts. Rather, the point is that altering environmental cues can lead to satisfaction with less food. For example, studies show that large-package size increases caloric consumption by some 22 per cent. Buying single-serve potato chips and small-size candy bars as opposed to "family bags" reduces consumption.
—Joe Schwarcz, “Pistachios reputed to have some uplifting health benefits,” Montreal Gazette, November 07, 2013
Ever finished a family-sized portion of crisps in one sitting?

That is because it’s easier to carry on eating rather than stop at one individual-sized portion.

This is similar to the pistachio principle — the idea that without a visual cue, such as empty pistachio shells or crisp packets, we’re not aware of how much we eat.
—Melissa Thompson, “Which diet tricks really work?,” The Mirror, August 29, 2011
2008 (earliest)
Another strategy is what he calls the pistachio principle. Given the same number of nuts (walnuts, peanuts, pistachios), people eat 45 percent less when they have to remove the shell.
—Sarah Wildman, “One-Bite-at-a-Time Diet Makeovers,” O, October 01, 2008