This is old news, but it’s back in the news and since it’s such an important topic, it is worth repeating. Dr. Eric Courchesne discusses the brain and gene basis of autism.
TL:DR: Brain imaging studies have revealed unusual growth patterns in individuals with autism, with overgrowth early in life. That early overgrowth is followed by a period of slowed or average growth, resulting in a normal or slightly larger brain volume in older children with autism.