A number of reports have been published indicating that head size in the first year of life has been found to be several percent greater in autistics, such as this latest one.
Media report linked here.
This is important and substantially competent research by this team. But there's some room for improvement.
Like most researchers they presume that autism is a "disorder" for which they are trying to find out what has "gone wrong". And so even when something good is found it has to be labelled as "overgrowth" of the brain.
And yet a remarkably similar progression of "overgrowth" has been found to be characteristic of high-IQ children (Gale, O'Callaghan, Bredow, Martyn 2010). And the 1982 published 1993 antiinnatia theory explained that whatever in low doses causes high IQ in higher doses causes autism.
You can see a very good presentation to the IACC about this "overgrowth", by the founder of this line of evidence, Eric Courchesne (between minutes 130-162 plus discussion to 174). He explains that it involves substantially higher numbers of neurons particularly in prefrontal. And that it is characteristic of "simplex" autism rather than of "multiplex" autism (defined as autism cases occurring more than once in a family). And furthermore, the advance over normal growth comes to level off after the first couple of years and indeed may be followed by a decline to below normal.
I've not looked into this research in great depth, but would provisionally suggest the following as reasonable explanations of these findings.
Multiplex autism could usually be due to a family sharing an indoor mercury vapor pollution problem from (mainly maternal) dental amalgams. EC notes that multiplex is much more common, which is in line with my conclusion that dental mercury- induced autism now accounts for 80% or more of all autism.
So it would be the simplex autism that is more a matter of too many antiinnatia genes, that is genes for high IQ, and which would also be genes favouring large heads and more neurons, which is in line with these observations. (The larger heads and more neurons would not cause the higher IQ but would be correlated with it.)
The subsequent decline would be readily explained by two processes. Firstly anyone with even fleeting acquaintance with autism can see that it soon leads to the autistic experiencing a substantially understimulating environment compared to neurotypicals. Such an understimulating environment can reasonably be expected to lead to the brain waning away somewhat. And meanwhile, for those autism cases caused by mercury, the mercury would continue to accumulate due to the defective detoxification, and the brain would consequently lose neurons due to that neurotoxin accumulation.