Pros and cons to both I would have thought. Not sure one is better than the other. Big family, plenty of company, never a dull moment, if siblings get on there's plenty of companionship. When parents get old(er), there's more support and more hands to help. Possibly and probably less disposable income, never a dull moment. If relations aren't great, not much escape until older.
Smaller families, in theory more attention from parents, financially life easier, practical things like trips and days out more do-able. If you don't get on with your sibling, that's a tough one. If an only child, ultimate attention but potentially lonely if parents arent sociable with other families, the child doesn't enjoy extra curricular clubs etc.