The number of junior classes in Welsh schools with more than 30 pupils has quadrupled in the past five years.
Official government statistics from July 2018 show that in 2017/18 there were 181 classes across Wales with more than 30 pupils, compared to 44 classes in 2013/14.
In 2013, the Welsh Government made it unlawful for infant schools to have one teacher in a Reception, Year 1 or 2 class in charge of 30 pupils or more, apart from certain 'statutory exceptions'.
The figures were released by the Assembly’s research service after a request by a Plaid Cymru AM.
Llyr Gruffydd AM, the Plaid Cymru shadow education secretary, said 'promises' are being broken.
However, the Welsh Government says it is funding initiatives to reduce class sizes.