Open the customer list. Sort it by revenue. Then sort it again by gross profit, cash collected, capacity consumed, and who owns the relationship.
The first list may show a great customer. The other lists may show a company designed around one account.
This is the contradiction. The customer did not take control in a meeting. The business gave control away one exception at a time: custom terms, special staffing, rush work, price protection, owner access, and capacity nobody else could buy.
Large revenue is not the problem. Unseen dependence is. A good account becomes structural risk when its loss changes payroll, delivery capacity, supplier commitments, or the owner's week before the company has a response.
A customer becomes concentration risk when losing or shrinking the account would force a material change in cash, capacity, pricing, or operations. Measure the dependency before celebrating the size.