Giant Pokémon have been a long-time staple of the anime, with the very first instance of a larger-than-life Pokémon appearing in Season 1, Episode 13, "Mystery at the Lighthouse." Over the years, there have been many more instances of giant Pokémon, and supersized critters have overtaken the battle scene with the advent of Dynamaxing.
Of those Pokémon who've found themselves tipping the (literal) scales prior to Gen 8, the anime usually has some explanation for their size, from pseudo-science to surprisingly plausible concepts, but not all examples are so clear-cut. Instead, many are left to the viewer's imagination.
Pokémon has employed a surprising amount of varied explanations to justify their kaiju-like Pokémon. In Season 1, Episode 19, there's a Tentacool that evolved with the help of Team Rocket's "Super secret stun sauce," that rather than stunning, accelerated its growth into a building-sized Tentacruel. In Season 7, Episode 25, a teleportation device accidentally supersizes both a Gulpin and Ash's Treeko when it malfunctions, and in Season 21, Episode 39, Faba accidentally creates a reverse shrink-ray, enlarging Sophocles' Togedemaru. There's also the case of a Caterpie in Season 8, Episode 49 who eats a mysterious blue candy, and ends up growing large enough that once it evolves into a Butterfree, not one, but two trainers can ride on its head!
Apparently, though, giant Pokémon are nothing new to the world of Pokémon, as ancient Pokémon often had cases of gigantism, as both Season 2, Episode 18's trapped Gengar, Alakazam and Jigglypuff and Season 8, Episode 12's Claydol prove. All four of these giant Pokémon had been sealed away by ancient peoples to prevent them from wreaking havoc with their large size and powers.
Of course, for each truly giant Pokémon, there are multitudes of Pokémon like Season 7, Episode 35's giant Whiscash or Season 2 Episode 17's giant Onix, which are simply implied to have grown larger than their usual counterparts, with no further hints. Similarly, every totem Pokémon is significantly larger than its non-totem counterparts, though it's not explained why.
Then there are those Pokémon who have kept their secrets for years. In the aforementioned "Mystery at the Lighthouse," Ash and Co. come across what is simply described as a "legendary Pokémon," but as any veteran of the "Who's that Pokémon?" segments can tell you, was just a supersized Dragonite. Given that the characters give no mention of the normal Dragonite in the episode, nor acknowledge it upon meeting (or catching) any other Dragonite, it stands to reason that they didn't recognize the Pokemon, possibly due to the thick fog coupled with the dark night. But how did this Dragonite get so large?
One Reddit theory posits that it was an in-universe early example of Dynamaxing, and may hint towards Dynamax dens existing outside of the Galar Region. Another theory takes a more out-of-universe stance, saying that the "legendary Pokémon" was originally supposed to be Lugia, hinting at Gen 2 the way that the first episode did with Ho-Oh. They point to the fact that the giant Pokémon comes from the sea, and the fact that its cry sounds eerily similar to Lugia's in the first movie, further theorizing that Lugia's design wasn't completely set in stone by the time the episode was being made, so instead, the animators subbed in the exceedingly rare and powerful Dragonite -- despite the fact it was a well-known Pokémon among fans.
But there are even more giant Pokémon out there without a hint of an explanation as to how they got so large. There's a recent example of this in Season 23 Episode 14, Golurk -- which marks the first time a giant Pokémon has been caught by one of our protagonists! But out of all these different reasons to go big, even with Dynamax, the process behind it is still a mystery to many, as trainers are usually far more concerned with how it works, rather than why.
Overall, gigantism in the Pokémon world has been going on for far longer than most realize from the unlikeliest sources -- and has even been capitalized on in-universe. Both Zoroark and Mismagius have been known to mess with trainers by creating illusions and dreams of giant Pokémon respectively, and Giovanni created an island of giant robotic Pokémon as an amusement park to help fund Team Rocket (or so we assume anyway). Perhaps Jesse, James and Meowth aren't too far off in their goal of getting Giovanni a Gigantimax-capable Pokémon after all...
