>>33358282Because she is an oppai loli. She's a young girl drawn like a teenager.
In terms of real stuff, Gamefreak and the anime purposely designed her to be sexually appealing because she was the first playable female lead at the start of a game generation and the first new female companion in the anime. They had to make sure she was a success, and what better way to do it then give her boobs?
Gen III also came out at a time where all the original kids were becoming teenagers and just going through puberty, so her body figure made sense for the target audience.
