Hmmm ok, most of the time I _do_ want a compressed version though since I can never rely on clients to do so.
Good to know what I'm dealing with though—thanks.
Someone in the Slack group suggested that I set the resize dimensions slightly off e.g. '395' rather than '400' to reduce the chances of the client uploading the correct image size.
I'll use this tip together with an image compression library like Matt suggested.