My Illogical Theory
Okay, I just want to clarify the fact that we have never seen Pokemon in real life.
Dunsparce is 1.5m, or 4'11'' in length. Length. According to here that's around the size of the average US female human, which even in length, is huge. Dunsparce doesn't even look that big, but apparently it is.
Charizard is 5'07'', or 1.7m, which is about the size of the average male (according to the link above), same as how Deoxys and Heatran are also the same height.
This may not have been Game Freak logic at all, Game Freak might have added these ridiculous sizes for the sake of easter eggs (for the people who notice, and get a laugh out of it) or they may have actually imagined it to be that height. But you've never seen Pokemon in real life, so you really have nothing to compare it with.
Like, you see that tree over there? In the distance, outside your window? It probably looks about the same height as you, but when you go closer to it it looks like the size of a Wailord. So don't jump to conclusions.
Hope I helped. :)
Source and the above link