Pokémon Size Variance

Pokémon GO was the first Pokémon game to introduce size variance. Whenever you catch a Pokémon, it will have a specific weight or height, based on the average heigh/weight of the Pokémon with larger or smaller Pokémon getting marked by XS, S, L or XL.

These had no visual difference and were just there as an interesting data point.

Size

XXS and XXL Pokémon

In Late 2022, XXS and XXL Pokémon were added to the game. This was a special size of the Pokémon and had their models and appearances change to appear small or large. They have a special animation when encountered and have their model scale down in all elements including AR shots, the Pokémon status screen, as your buddy and more.

These Pokémon also have a special element. When sent through Pokémon HOME to the main series games, they will be as big or small as possible, with XXS Pokémon getting a size value of 0 and XXL Pokémon getting a size value of 255. This means that they will be as big or as small as they can be, with this variance being shown natively in Pokémon Legends: Arceus and Pokémon Scarlet & Violet.
In Pokémon Scarlet & Violet, this also means these XXS & XXL Pokémon will be eligible for the Mini Mark & Jumbo Mark respectively.

XXS Larvitar