If you upload files bigger than 200px the system will resize them to 200px (width or height, if i remember right) and then the css will again queeze them into the avatar-holders.
Changing Avatar-Sizes based on the Resolution isn't a big deal, but this would only do a css-resize, not a real set of smaller avatars. So ... no benefit
The Style is only fitted to provide avatas up to 128px
If we want to increase them, there is a lot to change.
so perhaps sometimes later.
Moreover 400x300 will never come.
That's way to big.