I cant find anything on the wiki that makes this official, though some people say that it is true, and some say that there is no difference between going on a party of 1-2-3-4 people.
Anyone know if its true or not? Proof? Maybe someone from OOO/Sega staff could answer?
Proof, I have none. But in general people would say no. However, considering levels aren't always 100% the same, there's many factors that are involved that can give the illusion it makes a difference.
In a party of four, you're usually going to have at least one purpose who goes and chops all the bushes and breaks everything for those few extra crowns. If you were alone, you might not waste your time doing this, and those are some potential crowns that you lost.
There's no real guarantee that a monster will drop xxx amount of coins so you could have some days where you kill monsters that drop lots of coins, and others where you seem to be getting a lot of heat instead.