Some pupils are not being given enough food at Powys primary schools causing them to snack and lose concentration in lessons, an assembly member has warned.
Montgomeryshire AM Russell George said parents and governors had told him 11-year-olds were being given the same portion sizes as four-year-olds.
He claimed second helpings had also been banned and he intended to raise the issue with Powys council's leader.
The authority said its meals met Welsh government requirements.
Mr George said the issue was brought to his attention by a number of people across the county who he said felt the "extreme" school meals policy was leading to more snacking and bad dietary choices.
He said the policy dictated that all primary school children received the same portion of food, regardless of age.
And he said he had been told that second helpings of lunch had been stopped as the council tried to reduce waste.
"Parents are telling me that their children are coming home hungry, which then causes them to snack before their evening meal, all of which isn't lending itself to children leading healthy lifestyles," said the Conservative AM.
"Children of 10 and 11 years of age are clearly going to require different levels of nutrition to those of four and five-year-olds... this isn't rocket science."
One chair of governors at a primary school in the county claimed more parents were switching their children to packed lunches because they no longer felt the school meals were offering value for money.
He said he became concerned after noticing his own 10-year-old son was coming home from school hungry and that a number of other parents had raised the issue with him.
"I then raised it at a governor's meeting and was told that catering staff had been told that as of the start of this term additional food was not to be given," he said.
"I have spoken to the principal catering officer at the council and he said the rationale behind this was that he felt some schools were over-producing food.
"Also I think the thought is that by giving children seconds they're not going to have the balanced meal they should be getting.
"But what follows from that is that if you get wastage down to zero you have pupils going hungry as the portion sizes are the same for four to 11-year-olds."
He said teachers were concerned that it was leading to poorer concentration from some pupils.
"The council's saying that all the pupils should get the same portion sizes as everyone pays the same but I think it's leading to some parents feeling that they are no longer getting value for money and are giving their children packed lunches instead," he added.
A spokesperson for Powys council said dishes on school menus met the requirements of the Welsh government's Appetite for Life scheme, which aims to improve the nutritional standards of food and drink served in schools across Wales.
"As part of this scheme, there are recommended guidelines regarding the size of the portions. These portions have been analysed to ensure they meet the nutritional standards set out in the scheme," the spokesperson said.
"When planning school meals, the number of pupils who have school meals is always taken into consideration to ensure that school cooks do not over produce food and remain within their budgets.
"However, there will always be cases where food is left on the servery as pupils can choose whether they want a particular food on a school menu.
"If there is food left over, we have now advised school cooks to give out second helpings if there are pupils who want it.
"But we cannot overproduce food so that second helpings can be provided as this has a cost implication and also second portions are not taken into consideration when the menus are nutritionally analysed."
A Welsh government spokesperson said: "We would never want to see children having school meals going hungry.
"We've given primary schools guidance on portion sizes which cater for changing nutritional requirements as children get older. So a child in year six would have a larger portion than a child in reception."