The government's enthusiasm for opening more school sixth forms in England has been challenged by an MP who suggests exam results could suffer.
Sixth form colleges tend to offer a wider choice of courses
Pupils in the largest sixth forms got one top grade A-level, or equivalent, more than those in the smallest sixth forms, government data for 2006 showed.
Kelvin Hopkins MP, who obtained the figures in a written answer, said larger centres could run more courses.
The government said all types of sixth form provision should be high quality.
The majority of state-sector pupils are taught in sixth form colleges, but the government is investing funds in successful school sixth forms.
In 2006 there were 913 sixth form centres with more than 250 students and 178 with 50 or fewer.
Government data revealed the average points scored in sixth form groups of 50 students or fewer in 2004 were 200.
Sixth forms of 250 students or more achieved an average of 298 points.
These results were reflected in 2005 when the average points scored per student in the smallest and largest sixth forms was 205 and 316 respectively.
In 2006, the government changed the way it scored exam results but the pattern was the same.
Students in the smallest sixth forms attained an average of 571 points compared with an average of 847 for those in the largest centres.
Under this new system, one A-level grade A is worth 270 points while distinctions in a BTEC National Diploma, for example, score 810 points.
Mr Hopkins, who is a governor at Luton Sixth Form College, said the figures implied that sixth form colleges were doing better than schools.
He said: "I believe that sixth form colleges do an excellent job for the post-16 students because they have the widest possible range of courses as they are so large scale.
"At Luton Sixth Form College we offer 45 different A-levels and other vocational courses.
"In small school sixth forms where you have one teacher who is doing a second subject - that is not as strong.
"They are having to spread themselves very thinly to provide a range of courses."
Association of Colleges chief executive John Brennan said: "In order to make sure young people get the very best deal the government should be very cautious in encouraging more small sixth forms.
"These figures show that small sixth forms are unable to provide the quality of specialist teaching and choice of subjects which larger institutions can.
"Creating more would divert funding and students away from existing high quality provision."
A spokesman for the DfES said: "While it is true that the students in sixth form colleges generally perform better at A-level, differences are fairly small.
"It is important to ensure that all provision for post-16 learners, whatever their chosen pathway, should be high quality, which includes provision in school sixth forms.
"The government has introduced measures to increase the number of post-16 places in high performing institutions - schools and colleges - to increase the choice for local people of high quality places in a range of settings."