A survey of more than 1,000 parents also found 10 per cent of parents were routinely feeding pre-school children adult sized portions of meals such as spaghetti bolognaise and chicken nuggets.
Yet the findings from the Infant and Toddler Forum (ITF) highlighted a mismatch between the perception and reality as 73 per cent of parents were concerned that their child does not eat enough, and only a quarter worried that their child might become overweight in future.
More than one in seven Scottish five-year-olds is clinically obese or overweight when they start school, according to official data.
Gill Harris, a child and clinical psychologist from the ITF, said: “Most toddlers are naturally better than older children and adults at regulating their food intake. They usually only eat what they need and don’t overeat.
“However, portion size is critical. It’s one of the main ways in which, as parents, we can inadvertently override children’s self-regulation systems. Larger portions form our acceptance about what is an appropriate amount to eat and this becomes the “norm”.”
Some parents used food or drink between meals as a pacifier, with more than a third using snacks to calm children down, despite warnings this could encourage young children to rely on food to deal with emotions in later life.
A quarter of parents gave their child more than three times the recommended amount of sweets, while a third were pouring their children double the amount of fruit squash they should have.
Dr Donald Macgregor, of the Royal College of Paediatrics and Child Health (RCPCH), said: “28 per cent of children in Scotland are overweight or obese and portion size is one of the many factors that is contributing to the country’s incredibly high obesity rate.
“Other factors include advertising, the availability of cheap unhealthy food and lack of opportunity for safe outdoor play.”
Anna Strachan, Policy Officer of Obesity Action Scotland, said: “Portion sizes in the UK are increasing, fuelling the obesity epidemic. We know that larger portion sizes result in children consuming more calories than they need. The survey showed that the majority of children were routinely given portions of meals, drinks and treats that are far too big.
“Official Government information on portion size has not been updated for over 20 years.
“There is substantial variation in portion sizes offered by manufacturers and no consistency in communicating portion sizes to the public.
“Actions in public and private sectors to reduce size, availability, and appeal of larger portion sizes would help both parents and their children in Scotland to maintain a healthy weight.”