Economy of scale:
You build a toy car by hand. Each one is special parts and hourly work and is pretty expensive when everything is tallied up.
You start a factory that makes this toy car. Sure, the machines/building/employees are expensive, but now the parts are bought in bulk and 1000 toys can be made in 1 hour as opposed to, like 3. Production for the car because super low, even to the point of every car sold is more than enough to cover expenses.
Diseconomy:
The business gets bigger, more employees and cost bloat like rents end up making a big business cost more to keep running than a small one.
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