When you’re challenged as lying about your poker hand, your two options are: a) keep your hand secret, fold, and pay each challenger 5 gold or b) reveal your hand to prove that you are telling the truth and each challenger pays you 5X gold where X is the number of players in the game.
Other fines in the game are fixed, and do not individually scale with the number of players. In general, the number of players seems to be taken into account by the fact that the more people challenging etc the more money changes hands, naturally. Why is it different in this specific instance of challenging a truthful best hand?
The only thing I can think of is that, the more players there are, the harder it is to successfully be the best hand at the end, without lying, and so achieving that deserves more reward.