A small employer (or individual payer) is buying insurance, and the insurance company is going to need a good reserve because the maximum benefit is large relative to premiums.

A large company often self-insures the healthcare cost, and is buying health plan administration. If high costs are not observed, the reserves can roll over to future years. Even if they don't self-insure, the possibility can be used to negotiate lower prices, and having data on past claims can show a less risky pool, etc.

Self-selection.

https://en.wikipedia.org/wiki/Adverse_selection