Oversized classes of more than 30 pupils have increased in Devon since 2010, statistics from the Department of Education reveal.
More than 5,600 five to seven-year-olds in the South West are being taught in classes above the legal limit.
Devon County Council has said the increase is down to a rise in housing developments in the county.
In the Devon County Council area 435 infant pupils are being taught in over-sized classes, a rise of 94 or 28% in the four years.
A Devon County Council spokesman said: "There has been a big increase in pupils seeking primary places due to a rising birth rate and more housing development.
"Devon County Council has been working closely with heads and governors in key growth areas like Exeter and Exmouth to ensure sufficient investment in expanding schools to meet the need.
"These figures show we have been largely successful as they account for less than one per cent of the total primary population in Devon."
But a Department for Education spokeswoman said the rise was the result of demographic trends over the last decade.
"That is why we are giving local authorities £5bn to spend on new school places over this parliament – double the amount allocated by the previous government over an equivalent period," she said.
In Plymouth, the total has spiralled by 600% since the last general election. Plymouth recorded 651 infants in over-sized schools this year, up 558 or 600% in the period. In Somerset, 664 pupils are in classes of over 30, up by 132 or 25%. Torbay recorded no change, still having 62 pupils in over-sized classrooms.
Dorset has seen 563 pupils taught in 30-plus infant classes, an increase of 439 pupil or 354% since the last election.
In Cornwall, 282 infant pupils are being taught in over-sized classes, up 157 or 126% since 2010.
Labour claimed the figures showed that money was being diverted from state primary schools to fund Michael Gove's free school programme.
The party also pointed out that before the last election David Cameron promised "small schools with smaller class sizes".
But the Government blamed demographic changes for the rise and has pointed out that the free primary schools are also eligible for funding.
A limit on infant school class sizes was introduced by Labour in the late 1990s, stating that no more than 30 youngsters should be in a class.
Under the rules, there are certain circumstances in which schools can legally waive the limit, for example if a parent wins an appeal for a place.
Recent changes have also meant that classes can be made larger to take in twins, or the children of those serving in the armed forces.
The latest figures, published by the Department for Education as part of a snapshot of the make-up of England's schools, show that as of January 5,625 children were being taught in infant classes of more than 30 pupils.
Some 840 are being taught in classes "unlawfully" over the limit.
The most common reason for a class being lawfully expanded was for pupils moving into the area outside of normal admissions for a school and for whom there was no other school place within a reasonable distance.
Labour accused the Government of breaking a pledge to cut class sizes.
Ben Bradshaw, Labour MP for Exeter, said: "These are very large increases which show the pressures schools are now under. We know that children do best in smaller classes.
"Reducing maximum class sizes to 30 was one of Labour's historic pledges back in 1997, which we delivered. It's depressing we're now going backwards again under the Conservative-Liberal Democrat Government."
