From my understanding, the differerence is the companywide attitude to risk. A startup has a "grow at any cost, or maybe perish" attitude. If things go south, bankruptcy will take care of the leftover excess risk (barring criminal charges). A bigco cannot easily go bankrupt, even less a single department. There are tons and tons of capital to eat through if the accumulated risk is realized in cost. So a bigco has to do something about that risk somehow, because most of the company wants to keep what capital it has, only a small part of it really wants to risk things. So the only means to have a situation where you don't risk bigco for a single department is not making it a department. Make it a Ltd. in a holding or something.