I mean, it is obvious that you cannot sustain efficiency as you scale (Amdahl's law) but (1) $100M is not that crazy to be able to keep track of in your head, even for a single individual (I can imagine a successful real estate developer with a handful of ongoing projects and various other personal investments), and (2) in a high growth situation, it makes financial sense to sacrifice some economic gain for scale. In your original example, sure an investor would be better off, if they could actually find 10 good investments with zero cost, to spread their money, but very likely they'd be better off taking the big one and spend their energy raising more money.