In a large family, the parents are stretched. No one can deny this. The money is stretched. The children get less attention. In a small family, the children get individual attention. Just think about money. Money. Money makes the world go round. Money is the reason we exist. Large families cannot afford to spend much money on each. To get what each child needs. That is not such a problem in large families. It's only natural that children from small families are better off.
In a sense, staying as a small family is much more beneficial when it comes to money. If you have a huge family your expenses are naturally going to rise and things will be harder for you if you can not handle that kind of load. However, if you make good money and want to experience a big family then by all means, go ahead. But normally, a smaller family is better financially.
Yes, family units benefit from remaining small. The main reason is financial. The smaller the group, the less money it takes to keep the group going. It takes more money to run a large corporation than it does to run a small corner store. The more children you add to a family, the more expenses rise. Unless a person is wealthy, keeping the family small is a smart idea.
The smaller any organizational unit is the less noise is introduced into the system. For example a family of three (father, mother, child) benefits everyone involved by providing enough undivided attention for the child while reducing the stress of the parents toward the care of that child. In larger families, everyone suffers because attention, affection, and day-to-day resources become increasingly divided. This may lead to stress and resentment.
A small family means that family members are closer. It's far better to be known for who you are, rather than simply "child #7". In a day and age where more and more emphasis is placed on careers and entertainment, smaller families fight the mold by building stronger bonds. It's only natural.
All things have pros and cons. Small family units have some pros, like being more affordable and possibly being less chaotic. However, the cons maybe that unlike large family units, children in smaller family units may not develop an ability to effectively share space with others, or perhaps may not develop a strong ability to compromise. What works for one family may not work for another.