Some tours you can earn more medals than other people. The diving, bug and fishing are a few tours that you can earn more medals. You do need to come in first to get medals. The more people on the tours the more bonus medals.
Favorite tour is The Giant Dragonfly one. If you get gold that's 20 medals and than there are bonus medals for extras. Got 10 extra when I got first place with just one player. With more I can't remember that total because it's been awhile. I got more than 10 bonus medals when I got first place. Played that tour a lot and my extra player got a tour badge.
You earn more for playing competitive tours like the fishing and bug catching tours, but you don't earn more for playing cooperative tours like the hide-and-seek and scavenger tours if that makes sense. You earn more bonus medals the higher you finish among the people you play with. I think you get 10 bonus medals for 1st place, 7 for 2nd, 3 for 3rd, and 1 for 4th if I remember correctly. If you have fewer than 4 players, you only get bonus medals for the lower finishing positions, so with 2 players, 1st place gets 3 medals and 2nd place gets 1. So playing with more players is better.