Only four infant classes had more than 30 pupils this September, while numbers in oversized junior classes have also been slashed, according to Assembly statistics.
But education and lifelong learning minister Jane Davidson was criticised this week over how the government's 30-pupil limit is affecting mixed-age teaching.
Janet Ryder, Plaid Cymru spokeswoman, told the Assembly's education committee: "In some schools one year has to be split to meet the 30 in a class. Does this make parents question why their children haven't been allowed to move with their peers?"
But Ms Davidson said this gave "brighter" children a chance to shine earlier. "Estyn (the inspection agency) has said there is no problem with mixed-age pupils, nor evidence that class size makes a difference with older children, but there is a real difference with younger ones."
Statistics show a slight increase in the number of children in mixed key-stage groups, although average class sizes have fallen. Overall, there were 4,791 primary pupils in classes of 30-plus - down from 21,506 in September 2003.