Small lies can be doubted. Big Lies might actually be easier to sell.
Small, simple conspiracies can be uncovered and understood and prosecuted. A Big, world-bending conspiracy which blatantly turns everything on its head? It's so unbelievable no one would ever investigate it. No one except Trump.
I would add that the best way to tell a lie is to add some truth to it.
Then the whole thing can be dismissed, including the truthful parts.