Anglesey has overtaken Merthyr Tydfil as the area with the highest proportion of overweight or obese children, according to Public Health Wales.
The proportion for Wales (26%) is still worse than the worst region for child obesity in England (24%).
A total of 32.4% of four and five-year-olds are overweight or obese on Anglesey. Merthyr (16.4%) is still top for obesity.
More than 30,000 children were weighed for the annual survey.
The gap between the most and least deprived areas is narrowing but Public Health Wales (PHW) admitted there was still "much to do".
Merthyr Tydfil (32%) still had the second highest proportion of overweight or obese four and five year-olds, while Cardiff had one of the lowest, with 22.6%.
The report compared for the first time problems in rural and urban areas but found little difference.
A pilot study of children in the Cwm Taf health board area found that more than a fifth of healthy four and five-year-olds went on to become overweight or obese by the time they were aged eight or nine.
It also found 82.5% of children classed as obese at that age were still the same as they got older.
Dr Julie Bishop, director of health improvement for PHW, said healthy lifestyles for pre-school children should be promoted across Wales.
"There are no specific figures, but certainly a trend we're concerned about is the relationship between the amount of screen time - on tablets, phones, game and computers - and obesity," she said.
"We need to see children out playing more and we know there are always parents concerns about safety but they need to be running around more. Projects run by the likes of Communities First and mother and toddler groups can be supportive in helping parents."
Chief medical officer for Wales, Dr Ruth Hussey, said it was good to see obesity levels had levelled off, but there was still much to do and young people were eating too much sugar.
"The NHS alone cannot tackle childhood obesity," she said. "There is much that parents can do to ensure their children have the best possible start in life.
"Research suggests that breastfeeding is linked to a reduced risk of obesity in children, and parents can access information and support on this from health visitors."
The latest figures for Scotland for 2013 found 26.5% of two to six-year-old children were at risk of being overweight or obese.