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Almost 60% more children in their last year of primary school are classified as "severely obese" than in their first year, according to Public Health England figures for England and Wales.
The Local Government Association (LGA) said this shows children are becoming fatter as they go through school.
Every year, Public Health England - part of the Department of Health and Social Care which is responsible for the National Child Measurement Programme - produces figures on childhood obesity. This year, for the first time, they included a "severely obese" category.
But how reliable is this measure?
BMI (weight divided by height squared) is used to determine a healthy weight in adults. But for children under 18, because they're still growing, their BMI is plotted on a chart with reference to the heights and weights of a comparable population - in this case, the measurements of British children and young adults from nought to 23 years old collected between 1978 and 1994.
This measurement is known as the UK90 and is used to create a fixed point of comparison, so that while the population as a whole gets heavier, the threshold of what we consider healthy doesn't creep up.
Public Health England says any child whose BMI is in the 95th percentile of measurements for their age and sex, according to the UK90, is obese.
And those with measurements above 99.6% are "severely obese". Last year, about 60,000 children at the end of primary school in England and Wales were obese, of whom 22,000 were classed as "severely obese".
Clinically obese
Public Health England is responsible for overseeing national programmes on obesity.
But in the NHS in England, doctors tend to use a slightly higher threshold - they say a child is obese if they are in the 98th percentile of weights.
In other words, if you divided the range of all of the measurements in a population group into 100 equal chunks, the children whose weight fell into one of the top two chunks would be considered obese according to that measure.
Critics of the lower measurement say it brands a group of children as obese unnecessarily.
Christopher Snowdon at the Institute of Economic Affairs says too many assumptions were being made which "exaggerate the scale" of the obesity problem.
But Public Health England says it uses a lower threshold in order to capture children who are at risk of becoming under- or overweight or obese, as well as those who already fall into this category.
Paediatrician Dr Max Davie says: "No-one thinks it's the best or a perfect measure but it does correlate with the negative health outcomes associated with obesity."
Children with weights around about the 90th percentile and above start to be more and more likely to experience ill-health. And, on average, the higher you go up the scale, the higher the risk of harm.
Dr Davie says although both the 95% and 98% cut-off points for obesity are somewhat arbitrary, they still serve as a useful tool.
It might not be the perfect measure of when an individual child is at risk of ill-health, but it does give a way of comparing populations over time and around the country, he says.
"You can spend a lot of time debating where you put the threshold, but you have to be pragmatic," he says, and he believes there is enough evidence to suggest children with BMIs in the region of 90% and upwards are at higher risk of harm. It's also a simple enough measure that it can be applied to the whole population.
These are averages though - children develop at different rates, some may gain weight before a growth spurt in height, and there are ethnic differences too.
How does the UK compare?
The UK90 is not an internationally recognised measure and is rarely used outside the UK.
Researchers in 2014 looked at BMI data from six large-scale nationally representative surveys from Britain, the US, Brazil, Hong Kong, the Netherlands and Singapore. They found that in 2013 the UK had "considerably more overweight or obese children and young people than the average proportion among developed countries, for both boys and girls."
For both sexes there has been a more than 40% increase in overweight and obesity since 1980.
The researchers determined that the cut-off for obesity should be the 98th percentile of weights. They did not make reference to the "severely obese" category, however.