My thought process if I've seen great product managers oversee several products full time. Usually there's some kind of connection between them. If any one of them said, "Can you pay me a third just to oversee one of them" their boss would easily say Yes. But it helps to have the track record first.
Similarly, if a company (or product line) only has 2 or 3 engineers, who already know the problem domain, product management doesn't have to be huge.
Similarly, if a company (or product line) only has 2 or 3 engineers, who already know the problem domain, product management doesn't have to be huge.