Giant Pokémons?

They seem to be pretty common, at least in Anime and Manga (and being on both, they can pretty much be considered "canon").

The differences are not so slight to be considered normal (like a person being few inches taller or shorter), it's more like a mutation of some sort but no one ever reveals why or how those mutations happen, so what's your theory on it?

In my opinion they're just lab experiments escaped/dropped out in the wild.

In real life, it is possible for human activities to cause mutation in animals, isn't it? :b I'd imagine the same to happen in Pokémon world. Though, you see these kind of stuff in anime and manga very rarely, as people are implied to be careful about their environment and whatnot, haha.

If organizations like Team Rocket can figure out ways to get Pokemon to prematurely evolve, I don't think it's out of the question that someone's figured out how to turn Pokemon incredibly large.

Though, maybe it could have something to do with Pokemon's DNA, much like shiny Pokemon. Just like one in every 8192 Pokemon are shiny, maybe one in every X amount of Pokemon becomes giant. Gigantism is something that humans experience too, though to a much lesser degree than Pokemon, it seems. There have been examples of mini Pokemon showing up too, so maybe dwarfism exists as well in Pokemon species.