Wispa has become the latest chocolate bar to shrink down in size.
Manufacturer Cadbury says it is an effort to reduce the number of calories.
The treat is around 7% smaller than its previous size, and is being sold at the same price.
In 2020, the confectionary company cut the size of other popular bars Curly Wurlys, Fudge Bars and Chomps.
Damien McLoughlin is professor of marketing at the UCD Michael Smurfit Graduate Business School.
He told The Hard Shoulder this latest 'shrinkflation' is down to costs.
"The basic problem here is the price of ingredients for these bars.
"The price of cocoa in the last 15 years has been around $3,000 a metric tonne - and it's not far off that peak right now, at about $2,500 a tonne.
"So costs have gone up".
And he says companies have to be careful about giving consumers a reason not to buy their products.
"I haven't had a Wispa in quite a while... these brands rely on infrequent purchase.
"They don't have that many customers who eat one a day, or even one a week.
"Today you might have a Wispa on the way home or tomorrow you might have a Crunchie - or next Tuesday you might have a KitKat.
"What brands have to be very careful about in that kind of situation - in addition to the communications piece, the advertising - is they have to make sure not to give the consumer a reason not to buy."
Prof McLoughlin says people would be put off if the price went up.
"If it's 25% more expensive than another product, you won't buy it because you're giving a reason not to buy.
"Brand loyalty is a nice idea, but tends not to manifest itself in the way you think it might do".
And he says changing the price of products would drive people away.
"You tend to have a sense of what the category price is.
"A can of Coke is the same price as a can of Fanta, a tin of Lilt, a can of Sprite: they're all the same price, they're all in the same packaging.
"If you go out tomorrow and a can of Coke was 20c more expensive, you're going to buy Pepsi.
"Consumers tend not to know the exact price of things, but they have a sense of 'This is what the category costs'.
"If you buy a bar once a week... you have a sense that the price of it 'should be about this'.
"And if it's not, then you ask yourself a question: 'Do I really want to buy this?'
"And so if something is a little bit off what you expect it to be, then you're moving on to something else.
"That's a real problem for brands".