You can read all sides to the situation. Only children may have wanted to have siblings. Children of big families would have loved being an only child. Then you have some happy with who they are and what family they had. The issue is as diverse as the people.
Studies also differ in their evaluation of happiness, since that is such a personal factor.
Even the studies of physical and mental growth may harbor inaccuracies, based on the people studied. An advantaged only child may have hated being an only and be lacking in intellectual ability by choice or genetics, where as a child with many siblings may have loved the big family and excelled in academics and career. When it comes to growing up an only child, it can be tough. You have no older sibling to protect you from bullies, and no younger one to pick on. You have all your parents attention, which could be both good and bad, and you can be very spoiled, which in later years turns out to be a hard thing to be broken of.