Can you carry a 100 kg munchlax? Impossible! Well, Max carried a Munchlax in his back..How?! Even John Cena can't carry one!! This is because there are many errors in the physics of the pokemon world in the anime.
Well, I can't write all of them in one post. Check out this article in bulbapedia and we'll discuss about it here
Anime physics - Bulbapedia, the community-driven Pokémon encyclopedia