Additional downloads on screen shrink

Hi all,

I'm working with non-art directed images in a responsive layout, and
noticing that when shrinking the viewport, both Chrome and Opera are then
additionally requesting the smaller image instead of shrinking the already
downloaded asset.

Is this the intended behaviour? I couldn't see anything in the spec that
determines this behaviour, so is it up to browser vendors to determine how
to handle this? (Or the browser itself to look at network speed and make a
decision from there?)

I can see the need to request the new asset in art-directed cases (within
the Picture element for example), but in a simple srcset and sizes
situation is this additional download beneficial? Is the performance
implication of rescaling a large image offset by the network request?

Thanks
John