Well, it's really the memes that have spread the misconception of Sakuya having a small chest. As the fighters have established, Sakuya and Meiling have roughly the same chest size.

But, you know, memes.

Touhou 5 came before the fighter games and unlike the fighter games it was drawn by ZUN so, too bad it's cannon. In any case the fighter games are responsible for creating the meme. If you are to blame someone is your "cannon" Sakuya drawn by Twilight Frontier.