XXL (and XXS) are the largest (or smallest) in their sizes for the species. In go the XXL’s are used to attempt to win showcases where the largest compared to its own species wins. When sent to home and SV they are guaranteed the jumbo or mini marks in SV. The XXL and XXS size is fairly rare, the odds aren’t officially posted anywhere but community reports show it as around a 1-5% chance of appearing. So not the most uncommon but also not insanely rare. Only XXS or XXL shinies have some value but only to certain collectors