LUCARIO YOU ARE SO SMALL
IS FUNNY TO ME.

While everything the Pokédex tells us should be taken with a grain of salt, I do think some of the heights are misinterpreted in this picture.

First, in case of Dratini and all other snake-like pokémon like it, I think their listed height is actually their length, measured from their head to the tip of their tails. Arbok, for example, has a "height" of 3,5 metres, which sits somewhere between the average lengths of an Indian cobra and a king cobra.

Second, horses (and other members of its family such as zebras) and also dogs have their height measured at their withers, where the neck meets the back. Rapidash's height, for example, is listed as 1,7 metres… which is a common height among larger horse riding breeds. For Ponyta, as a baby/infant horse, to be 1 metre tall is also pretty realistic.

There are still quite a few pokémon sizes that don't make sense at all, but at least not all are as outrageous as they might seem at first.