Since the beginning of the game, each Pokemon has a specific weight in pounds or kilograms, based on the version (language) of your game. This can be viewed in the Pokedex, and in HeartGold/SoulSilver, you could compare your trainer and that certain Pokemon on a scale. Generation III, with Ruby and Sapphire versions, started using two moves which would determine damage based on the target’s weight.
Low Kick actually started from Generation I, having a constant base power. In battle, weight plays a small part in battle strategies, as weight does not determine the Speed stat. However, the weight of a Pokemon foreshadows its Speed; heavy figures tend to move slower in battle, with a few exceptions.
In Generation V, more moves are being created that deal with weight. A common move used is Grass Knot, which is a Special attack. Since all nations except for the United States use the metric system, weight categories are separated into six groups: under 10 kg, 10-25 kg, 25-50 kg, 50-100 kg, 100-200 kg, and over 200 kg. Grass Knot (Grass-type and special) and Low Kick (Fighting-type and physical) both use these categories, and its power is predetermined to do 20, 40, 60, 80, 100, or 120 damage.
Generation V introduced Heavy Bomber, the Steel-type move, and Heat Stamp, the Fire-type move, both of which are physical. Contrary to Grass Knot and Low Kick, these moves factor in the user’s weight. The added twist is that power is determined based on how heavier the user is than the target, which is categorized into five groups: if the target is 50% of the user’s weight, 33-50%, 25-33%, 20-25%, and less than 20%. The damage is massive: 40, 60, 80, 100, and 120, respectively. The only weight-based status move that exists right now is Body Purge, a Steel-type move which doubles its Speed in return for halving its weight. Hold items were made to influence or discourage weight-based moves. For example, the Float Stone halves a Pokemon’s weight, discouraging Low Kick or Grass Knot.
Currently, Hidden Abilities in Japan are titled Light Metal and Heavy Metal, both of which can be found in Dream World Pokemon. These abilities half or double the Pokemon’s weight, which could be beneficial or harmful, based on how you use them in battle. We are still waiting for the Pokemon Dream World International to release new Hidden Abilities and update the variety of Pokemon that can be found.
In the TV series, the gym leader Gardenia utilized Grass Knot in her battle against Ash, which shows that the creators are always trying to make players aware of their new moves for even better strategies. With the proper ability and hold item, a Pokemon can even cut its weight to a quarter of what it was before. Experimenting with weight can definitely add twists to a battle, and it is also something to be aware of when battling.