It's a Meganium, the final evolution of Gen II starter Pokemon Chikorita. It's not legendary or mythical (unless this one is special somehow). It has appeared in the Johto games, Emerald, ORAS, and SM, and can be transferred to the games in Gens II - VII. It cannot be transferred to SwSh, at least not yet. There are no concrete reasons as to why it's so big. Some theorize Dynamax, others speculate a new gimmick that the game will bring with it.
Edit: As you said, it's a special sort of Pokemon called an "Illumina Pokemon". Not much else is known apart from the name and that it's a phenomenon supposedly only found in the Lental region, where New Pokemon Snap takes place.
Hope I helped!