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.