Calories

Calories are a value inside of every edible item in the Pokemon world which have many effects on Pokemon, both visually and functionally
==Visual Effects of Calories==
When Pokemon consume a food, such as Poffins they gain "Calories" not only as a value but also visibly on their waistline, an example provided in the provided image. While Pokemon can gain weight anywhere they tend to show it most on their bellies, developing heavy haunches and forming fatty navels. Of course weight will eventually show in other places as the Pokemon continues to pig out on food.
=== Weights go as such: ===
Slim -> Average -> Tubby -> Potbellied -> Fatty -> Glutton -> Obese -> Flabby -> Blob -> Immobile
Pokemon that gain weight rarely choose to lose it, resulting in a world of portly Pokemon who often bump bellies unintentionally.
== Functional Effects of Calories ==
Calorie gain, and therefore weight gain, have a simple to understand effect on battling, with weight scaling exponentially with a drop in Speed and a gain in Physical Defense, having no effect on Special Defense. This also has logical effects on certain moves, such as Body Slam and Tackle. Your size may also lead to certain corridors becoming harder or impossible to squeeze through.