Over the past 10-20 years, kids' waistlines have been expanding, which can cause health problems.
The study compared the waist measurements of kids now, with young people in the 1970s - and they've definitely got bigger.
In 1977, just one in 10 boys and girls were overweight.
Just 2-3% of kids were really fat, or 'obese', in 1977, but this has shot up to nearer one in 10 now.
Being fat can lead to problems like diabetes and high cholesterol in later life, researchers warned.