Australians' unhealthy penchant for pizza and cake could be helping to fuel the obesity epidemic as portion sizes grow to gigantic proportions.
The average size of a slice of cake now contains almost 1000 kilojules more than it did two decades ago, according to a new study.
Research by The George Institute for Global Health reveals portion sizes for a large proportion of Australia's most commonly consumed junk foods have risen significantly.
Data from the 1995 National Nutrition Survey and the 2011-2012 National Nutrition and Physical Activity Survey were analysed to track the changes in portion sizes of discretionary foods.
Both pizza and cake recorded a 66 per cent increase in the average number of kilojules consumed in one sitting.
Typical portion sizes for ice cream, sausage, processed meat and wine also increased significantly.
Serves of ice-cream eaten by women had increased in size by nearly a third.
Given discretionary foods contribute about a third of a person's daily average energy intake, the "worrying" surge in portion sizes has become a gigantic problem, says lead researcher Dr Miaobing Zheng.
"Over the past two decades we found that foods which provide very little nutritional benefit have surged in size, and this is helping to fuel Australia's obesity epidemic," Dr Zheng said.
"We had expected portion sizes to have grown but we were still surprised by just how much," Dr Zheng said.
However, portion sizes of pastries, snack foods such as potato chips, popcorn and corn chips, and potato fries had actually decreased.
The reason for this is unclear but it could be that these foods are typically considered unhealthy and people consciously try to eat less of them.
Dr Zheng warns the true picture of what Australians are eating could be "much worse" because people often under-report the amount of food and drink they consume.
Co-author Beth Meertens from The Heart Foundation said it's important the community understands how portion size influences the amount of food they eat and affects their health.
"Healthy eating is both the type of food and the amount of food that we choose every day," Ms Meertens said.
She said food companies, governments and health bodies can do more to promote healthier portion sizes of these discretionary foods.
TIPS TO CURB PORTION SIZE
* Use smaller plates
* Choose entree size when eating out
* Share that slice of cake with a friend
* When shopping, don't buy supersize.