@Fizzcube: My thoughts on the writers of the anime is:
Considering the time it takes to thoroughly play any of the games, and how quickly they come out with episodes involving new Pokemon from those generations shortly after the games come out, it is very likely that they already started writing those episodes with minor prior knowledge about the games beforehand. They most likely just had development notes from the artists and writers of the games, and none of the statistics that the games have (like height values, for instance), leading them to base the sizes around the images comparing those images to the main character's height. In other words, they make an educated guess, and have no clue how strict those numbers are supposed to be.