Explain in ACP Attachment settings images get resized (at client-side)

In ACP > Attachment settings there is the setting maximum image size. It's not really explained here that images get resized client side before upload when trying to upload an image with exceding dimensions. (People are requesting an extension for resizing image attachments. https://www.phpbb.com/community/viewtopic.php?f=496&t=2275566)