Why you might not be able to use the same measure to compare the size of two business?

Sales as an indicator: Two business may have the same sales but one has a low margin and low input costs making it a smaller business

Number of employees: Two companies have 100 employees but one manufactures large complex objects (EG: drilling rigs) The other is a store in the mall. The store in the mall will be the smaller company.

Profit: Many large companies loose money, many small companies make a profit.

Assets: A company may lease all its production material while the next owns its assets making it a larger company.

And so many more examples

The key is to use several indicators and make sure you are comparing Apples to Apples.