Basically this mod adds "realistic" (highly doubt it) fatness in minecraft. How does steve eat so much food so quickly without any consequences?!?!
The more you eat, the fatter steve gets, well, more complicated than that..



To learn all about the datapack craft the healthness book by throwing an apple and a book on the ground

REQUIRES PEHKUI MOD TO WORK!!

More in-depth guide:
Getting to full hunger (meat bars) you start to grow, it isn't that noticeable.
Getting to around 15 hunger (or 7.5 meat sticks) you begin to shrink. Running or swimming will decrease this amount quicker.
As you grow, you become more thirsty and generate waste. You don't know when you are thirsty, so you must drink regularly. To get rid of waste go on an EMPTY cauldron, crouch and move slightly on it. If you don't drink it will be harder for you to get rid of waste, and if you don't get rid of waste the rate at which you grow increases.
Getting bigger has its advantages, you are stronger and take less damage, but you will be bigger and slower.
Getting smaller also has its uses. Although you are weaker and take more damage, you are faster and could fit in smaller gaps (such as through iron bars)

Other important things to note:
1. STAY AWAY FROM ENDERMITES, they can go inside you and kill you, and other mobs. This will make them multiply, which can be dangerous.
2. USE RANGED ATTACKS FOR HUSKS. Husks can now shrink you at a very fast rate, while making them stronger.
3. To prevent both endermites and husks special abilities, craft an immunity chestplate by throwing 8 magma cream and an iron chestplate on the ground.