Mars, which makes the chocolates, said in March that it had taken the decision in order to help people afford the product as the company combats rising commodity prices.
The Office for National Statistics has studied the impact of "shrinkflation" where products are shrunk but without a cut in price.
More than 2,500 products have shrunk in size over the past five years while still being being sold for the same price, according to official data.
The phenomenon has been seen in products such as Toblerone, where the gaps between the "peaks" have got wider.
Shrinkflation affects candies and chocolate more than any other category of goods, the ONS said, but has a negligible effect on overall inflation.
Packets of Maltesers have shrunk from 121g to 103g, a reduction of 15%.
Dozens of chocolate bars and candies have already got smaller.
Summing up its findings, the ONS said: "No, you're not imagining it - some of your favourite candies really are shrinking".
The ONS assures the public it catches this form of inflation because its hundreds of price collectors around the country always check the size of an item as well as the price when they are rounding up information for each month's inflation figures.
The figures come following high profile cases like Toblerone a year ago, resulting in the downsizing of a bigger bar from 400 grams to 360 grams.
'But our analysis doesn't show a noticeable change following the referendum that would point towards a Brexit effect. It said the import price of sugar for European companies is close to its lowest level on record, while cocoa prices have fallen sharply since reaching a high in 2015. The prices of both white sugar and cocoa are now nearing five year lows having peaked in July last year.
In all the ONS said some 2,529 products it monitored in the United Kingdom shrank in size between January 2012 and June 2017. "Furthermore, others had been observing these shrinking pack sizes long before the European Union referendum, and several manufacturers have denied that this is a major factor".