>>36469849because dex entries including weight & height are extremely inconsistent. Especially when taking anime into factor. It's difficult to say which one is "canon" but I think there is some healthy variance between Pokemon in terms of size and weight. I mean if you have a Pikachu genetically it could be a much stronger & muscular Pikachu so it's very likely it will be bigger than a runt Pikachu. Take into factor the Pokemon's health & diet. If you feed your Pikachu junk and never take it out for walks or let it exercise it will get fat compared to a healthy Pikachu.
tl;dr Pokemon weight & sizes are bullshit and no one cares. it's a children's game and tv show for fucks sake.