"I'll just have a few"

Start with the intention, because it's real and it fails every time, and the reason it fails is one of the tidier findings in the psychology of eating.

Andrew Geier, Paul Rozin and Gheorghe Doros, in 2006, described what they called unit bias, which is the tendency to treat one unit of something as the right amount to have, whatever the size of the unit. They tested it by leaving out food in public places with a sign saying help yourself. Sweets in a bowl, with a small scoop one day and a large scoop another. Pretzels, whole one day and halved the next. People took a scoop, or a pretzel, and the scoop or the pretzel was what they took, so on the large-scoop and whole-pretzel days they took much more, by weight, without anyone taking more scoops or more pretzels. One unit felt like the right amount, and the unit was whatever they'd been given.

An earlier book in this series, on eating past full, uses this study for plates. I'm using it for bags, because the bag is the unit. When you open a sharing bag of crisps, the amount that feels like the right amount isn't a handful. It's the bag. Not because you've decided that, but because that's what a unit does. It presents itself as the amount, and the hand goes until the unit is finished, and a sharing bag is a hundred and fifty grams. "A few" was your intention. "The bag" was the unit's, and the unit was set by someone who wanted to sell a bigger bag.

The finding is stronger than it sounds, because Geier's participants weren't stopped from taking more. They could have taken two small scoops on the small-scoop day and matched the large-scoop day. They didn't. One scoop was the amount, and the same is true in reverse. If you put a handful of crisps in a bowl, the bowl becomes the unit, and the bowl is the amount, and you'd have to get up to have more, and mostly you won't. The intention to have a few can't win against a bag. It can win easily against a bowl, because the bowl is a few.

So when you say you'll just have a few, you're describing an intention that has never once survived contact with a sharing bag, and that isn't a fact about your character. It's a fact about units. Change the unit and the intention gets a chance.