Mars has blamed rising costs for its decision to shrink the size of Maltesers packets.
The firm has reduced the weight of the product's 121g pouches to 103g, a cut of almost 15%.
The decrease was spotted by food retail expert Steve Dresser, who also noticed that packets of Mars' Galaxy Counters have been dropped from 126g to 112g in weight.
In a statement, Mars said rising costs forced it to decide between increasing the price of Maltesers pouches or reducing the size of the product.
The firm said: "Like all chocolate manufacturers, we have seen the cost of raw materials rise and, while we try to absorb these pressures as much as possible, sometimes we have to make the difficult decision to reduce the size of some of our products so our consumers can continue to enjoy an affordable treat."
Mars added: "Our focus is always on offering consumers our great tasting, high quality chocolate brands at the best value for money."
In recent years the firm has come under fire for reducing the size of Mars and Snickers bars.
Earlier this month, Walkers crisps and Birds Eye announced price increases which were blamed on the falling strength of the pound since the UK voted to leave the European Union.
The pound has fallen about 16% against the US dollar since the Brexit vote, making imported goods more expensive.
The drop has prompted Tesco chief executive Dave Lewis to call on suppliers not to hike prices just so that profits look good to investors.