The government's statistics body said more than 2,000 products had been scaled back in size since 2012 in a process it calls "shrinkflation" because consumers get less for their money.
The ONS collects prices of products each month from shops around the UK and also checks the weight. More than 2,000 food and drink products had decreased in size since January 2012, it said.
The ONS said shrinkflation was most notable in sugar, jam, syrups, chocolate, and confectionery products.
Businesses blame changes in the cost of raw materials, such as sugar or cocoa, for the reduction in pack sizes. They say reducing size prevents them from having to put the cost up.
Food giants including Mondelez, which owns Toblerone, and Mars, which owns Maltesers and Galaxy, have blamed rising raw material costs for the downsize.
Maltesers reduced its pack size from 121g to 103g, and Galaxy Counters shrank from 126g down to 112g, according to the BBC.
Toblerone, meanwhile, faced a consumer backlash when it removed some triangles from its bar, reducing it in size by 10%.
Other items to have reduced in size without a corresponding price drop include McVitie's dark chocolate digestives, according to reports.
The ONS said the prices of cocoa and sugar, which have "fluctuated considerably in recent years", have both fallen in 2017.
However, its figures don't appear to show this has translated to a price drop.
Some analysts have warned that the drop in the value of the pound since the Brexit vote may have added pressure to costs.
But the ONS said its analysis "doesn’t show a noticeable change following the referendum that would point towards a Brexit effect".
It noted: "Furthermore, others (including Which?) had been observing these shrinking pack sizes long before the EU referendum, and several manufacturers have denied that this is a major factor."