>>58041188
Probably just a simple Godzilla/Kaiju movie reference. I guess the Japanese like that sort of thing. There was also an episode in the first season (I think) where a giant Gengar fights and giant Alakazam, and they're both put to sleep by a giant Jigglypuff. And of course, there's the episode where a barrel of ??? substance lands on a Tentacruel and turns it into the size of a building. AND we have that herb in the newer games that can cause Pokemon to grow.
At any rate, I like the idea that while incredibly rare, there's house-sized Pokemon wandering around in more remote areas.