The amount of investment is irrelevant to the chance of that person running away. I'd say for many things a higher investment makes it even unliklier that a person vanishes, because the amount of money he can gain from the investments is enough to make a living. A site that accepts microinvestments will most likely be only a hobby.
That is nonsense, you dont look from their perspective you look for your pespecitve. A street hustler can steal the same way 1$ like Bernie Madoff stole millions of $.
The question remains which hurts the most losing 0.001
BTC or losing 100
BTC.
Also dont underestimate the scammers around bitcoin. Even if he can make a fulltime income with it, he can still run away with it if there is no way of detecting his identity.
Also running a microinvestment site can be profitable aswell if enough people invest in you, and even if it turns out as a scam, the individuals lost much less / capita , than if it were a closed investment with 10 people investing 100
BTC, the /capita loss would be big.