A tin of Cadbury's Roses now weighs 850g, down from 975g, while a tin of Heroes now contains 800g of chocolate 150g less than last Christmas. The reduction is the equivalent of 11 fewer chocolates in each tin.
However, shelf and wholesale prices have remained flat effectively passing on a double-digit price hike to customers.
Cadbury had decided to shrink the tins because of rising commodity costs, a spokesman said. "However, we still believe that our tins represent excellent value for money," he added.
Nestlé's Quality Street tin will remain at 1kg after it shed 100g last year, while Mars' Celebrations tubs remain at 855g, after shrinking from 975g two years ago.
Tesco, the first supermarket to stock the tins this year, is advertising them at half price for £5, the most popular promotional mechanic last year.
One wholesale trading controller said shrinking the pack was a better choice than price rises. "It's more important to maintain a successful price point based on the current climate of rising raw material costs," he said.
Consumers would notice the difference, but few would be deterred from buying, he added. "Virtually everybody has changed pack sizes. Consumers understand the cost of food now."
But Lucy Yates, principal policy advocate at Consumer Focus, warned companies to be honest with their customers about the price hikes.
"Consumers aren't always easy to fool they can see that paying the same for fewer chocolates is effectively the same as a price increase. Companies need to remember that shrinking size but not price could damage consumers' trust in the brands they love."
Cadbury has meanwhile abandoned a trial to move packaging over to recyclable cardboard boxes.
In 2009, it began selling Roses and Heroes in so-called 'cartins' at 500 Tesco stores. At the time Cadbury said the move would cut packaging weight by 45% and save more than 200 tonnes of steel.
Last year the trial switched to selling all 950g boxes of Heroes in cardboard as a Tesco exclusive, doubling the amount of cartins it used.
However, the company confirmed it had returned to tin for 2011 after consumers snubbed the eco-friendly alternative.
"We spoke extensively with consumers to understand their reaction to cartins last year but the feedback was disappointing," a spokesman said.