When a business produces multiple products instead of specialising in one, it's cheaper for one business to produce many products than it is for many businesses to produce one product each
A business that already has people and an infrastructure in place will be more efficient at producing an additional product than a new business, specialising in only that product, will be
Existing businesses are also able to benefit from brand loyalty - people already know the company's brand, so they are more likely to buy other products that they make
Economies of scope allow businesses to charge lower prices due to lower unit costs
Poor coordination makes a business less efficient, it's hard to coordinate activities between different departments in a big firm
Communication is harder in a big business, it can be slow and difficult to get messages to the right people, especially when there are long chains of command
It can be hard to motivate people in a large firm, in a small firm managers are in close contact with staff, and it's easier for people to feel like they belong and that they're working towards the same aims
It's easier to manage and control a smaller business, so a large business might split up into several smaller ones and focus on making each one profitable