Weight has been a part of Pokémon since the very first games, appearing in all Pokémon's Pokédex entries. However, from Ruby & Sapphire, two moves started using it in their calculation of damage. This then got further enhanced in Black & White with more moves which use weight as well as moves, abilities and items which then manipulate the user's weight. This is all based upon the weight within their Pokédex entries

Weight Manipulation

The first part we'll cover is the weight manipulation. This allows you to utilise the moves and abilities in order to change your weight. This can only be done in factors of 2 (halving or doubling) and some methods like an Item + Ability are in fact stackable so you can cause a Pokémon's weight to be cut to a quarter of its actual weight. There are various methods for weight manipulation and are as follows.

Reducing Weight

Attacks

Name	Type	Cat.	PP	Att.	Acc.	Effect
Autotomize			15	--	--	It cuts off useless parts of its body. It sharply raises its speed and makes its weight lighter by 100kg

Abilities

Items

Picture	Name	Effect
	Float Stone	A very light stone. If held a Pokemon's weight becomes lighter.

Increasing Weight

Abilities

Heavy Slam & Heat Crash

Heavy Slam& Heat Crash were both introduced in the game in Pokémon Black & White. They work on a similar manner to Grass Knot & Low Kick, but instead factor in both the user's weight and the opponent's weight and as such, can be mercilessly manipulated using the above weight manipulation methods. These moves will fail against Dynamaxed Pokémon.

These moves have got a set amount of Power in which it can inflict, all within a special band, not of weights, but of percentages in comparsion to your weight and your opponent's weight. The set-up is as follows.

Below are the full details for these two moves;

Name	Type	Cat.	PP	Att.	Acc.	Effect
Heat Crash			10	--	100	It slams into the opponent with a flaming body. The more heavy than the opponent it is, the stronger the power.
Heavy Slam			10	--	100	It attacks by striking the opponent with its heavy body. The heavier it is than its opponent, the greater the power.

Grass Knot & Low Kick

Grass Knot & Low Kick are two rather unique attacks. They are unique in the manner that their base power is not contingent on the Pokémon using it, but rather on how heavy and bulky the target is. Low Kick was originally introduced with a basic power in Generation I, but it wasn't until Generation III where it started being based upon the opponent's weight. These moves will fail against Dynamaxed Pokémon.

These two seperate attacks incorporate the same functionality as eachother, but Low Kick is Physical while Grass Knot is Special. This factor requires you to completely factor in who you are using it against as well as the type.

Now, how is the base power calculated? There is a rather basic set up. It is based on your target's weight. This alone determines how strong, or how weak the attack is going to be. With as little as 20 Base Power, it can go as high as 120, making it a very powerful attack if used against the right team. Below is the set up of how it is calculated, and click the weight descriptions to go to a list which lists the Pokémon within that band

Below are the full details for these two moves;

Name	Type	Cat.	PP	Att.	Acc.	Effect
Grass Knot			20	--	100	The user snares the foe with grass and trips it. The heavier the foe, the greater the damage.
Low Kick			20	--	100	A powerful low kick that makes the foe fall over. It inflicts greater damage on heavier foes.

Sky Drop

Sky Drop is a move that has the Pokémon pick up its opponent and be in the air. As of X & Y, this move stopped working on Pokémon that weigh over 200kg. If the target Pokémon weigh more than that, the move will fail