Scaling means output growing faster than the cost of producing it.
Why most service businesses do not scale
Doubling the work generally requires close to double the people and vehicles. The cost rises with the revenue, so margin stays roughly flat.
That is expansion rather than scaling, and it is a perfectly sound way to build a business. Expecting margins to improve with size, and finding they do not, is what causes the disappointment.
Where genuine economies exist
Overhead spread wider. Premises, software, insurance and administration do not double when revenue doubles, so overhead falls as a percentage.
Buying power on materials at volume — see vendor management.
Route density. More customers in the same area means less unbilled travel, which is one of the few real efficiency gains available to a mobile trade — see tickets per route.
Where costs rise faster than revenue
Supervision. Beyond a handful of people, someone has to manage, and they are not producing — see organisational structure.
Coordination. Scheduling and dispatch grow more than proportionally with headcount.
Quality. Consistency across a larger team requires systems that were unnecessary before — see quality control.
These are why margin frequently falls during growth rather than rising.
The constraints that bite first
Cash, because growth consumes working capital before it produces profit.
People, which is usually the binding constraint in a skilled trade.
The owner's capacity — see key person risk.
The alternative to getting bigger
Better rather than larger: higher margin work, higher average job value, more recurring revenue. It produces more profit without more of the problems above, and it is available to a business that has decided it does not want to grow headcount.