Answer:

Bridgewater and RenTech have different business models.

RenTech is essentially a collection of math and physics PhDs, along with some administrative personnel. It only manages its principals’ (employees’) money.

Bridgewater manages its own internal funds, as well as those of outside clients. As soon as you manage outside clients’ money, you need a lot more staff to meet certain legal and regulatory requirements that pertain to managing outside investors’ cash. Additionally, Bridgewater has developed some kind of complex people tracking system, which, it appears, at least based on reports of the system from outsiders, requires a lot of people to manage.