Mars says it has shrunk its Mars bar from 58g to 51g and its Snickers bar to 48g, to meet its public health targets.
The calorie count in the bars has now dropped from 260 to 229 and from 280 to 245 respectively, according to reports in The Grocer.
The move follows Mars’ pledge in 2012 to reduce the calorie content of its single serve products to 250 by the end of 2013.
A spokesperson from Mars said: "As part of our global commitment to promote responsible consumption, and as a signatory of the Responsibility Deal’s calorie reduction pledge, we [said we would] ensure that all of our single serve chocolate products would contain no more than 250 calories per portion.
"To meet our commitment, having taken product reformulation as far as we can for now without compromising the great taste, we have reduced the portion size of Mars and Snickers to bring down the calorie content."
In October, confectionery brands faced renewed pressure to take responsibility for their strategic positioning at check outs, after the government’s new public health minister Jane Ellison termed the issue "an area for action".
Ellison said food promotion has been identified as an area for action under the Responsibility Deal, and discussions will be happening between the government and the food industry on actions it might take to reduce exposure to marketing and promotion of less healthy food.