I believe there is a point after which the higher dosage doesn't change much, but the duration of the trip.
I have noticed the same effect with mushrooms (truffles) where basically doubling the dose didn't do much.
I'm not even sure it lasted that much longer, because of course you don't take them in a fixed reproducible setting where you could correctly compare stuff.
I think there is a lot of bullshit around those substances, pushed by shamanic type of people who just invent and repeat nonsense to appear unique/interesting.
The "reports" on Erowid, while interesting in a way, are also so full of nonsense that it's largely irrelevant as a source...