This is not a bug, it's a feature of Magento.
When you upload an image for a product with specific image sizes, like 1200x1200 and you use it on the thumbnail with the size 60x60 and small image like 200x200 and large image like 600x600. It creates cache images according to your given size.
For 2nd question:
You can stop it by creating. And why anyone ...