A Pokemon's size will actually have some meaning in Pokemon: Let's Go.
Pokemon: Let's Go, the upcoming Pokemon game for the Nintendo Switch, is borrowing several mechanics and features from the mobile game Pokemon Go. The catch screen and mechanic are similar, and players will eventually have the option of transferring Pokemon to Professor Oak in exchange for candies that can be used to strengthen a Pokemon.
Another unusual mechanic coming from Pokemon Go to Pokemon: Let's Go is that each individual Pokemon has their own individual height and weight. The individual height and weight are largely just for flavor in Pokemon Go, a way to give out a few more badges and add a cutesy line of text when appraising Pokemon. But in Pokemon: Let's Go, the size of a Pokemon actually plays an important role n the game.
Pokemon: Let's Go uses a similar XP mechanic to Pokemon Go, only the Pokemon receive the experience points instead of a player. Players can increase their bonus multiplier by reaching certain criteria. For instance, a curve ball throw or a nice throw will increase the amount of XP a player's Pokemon gets.
Another thing factored into XP is how big or small the Pokemon caught is. The bigger the Pokemon, the more XP a player will earn for catching it. Two Pokemon could be of the same species and level, but a player will get more XP for catching the Pokemon that weighs more than the undersized one.
These differences will also be reflected in the overworld map. An undersized Pokemon will have a blue highlight surrounding it on the mpa, while a larger Pokemon will have a red highlight. If you're trying to gain XP for your Pokemon, you'll want to stick to the red highlighted Pokemon (or the Pokemon with no highlights at all) to get the most XP while grinding.
This is actually an interesting change from traditional Pokemon games, which have always depicted Pokemon as being the same weight and height. By adding some variation to a Pokemon's size, they've made the Pokemon world feel a little more realistic, which seems like one of the secondary goals of the Pokemon: Let's Go games.
Pokemon: Let's Go, Pikachu and Pokemon: Let's Go, Eevee will be released on December 16th.