How can I reduce the file sizes of the original versions of uploaded image files?
Interesting question - this should be a standard option on community distributions to avoid the space issue you raised but you're right that the focus seems to be on image styles as a solution..

Two ways to handle this


More custom way would be to add a hook_form_alter to your form that would add a submit handler to resize the image file after uploaded to the correct size -- if you are able to do image styles than your server already has the image libraries needed to do this.
An easier way would be to use an uploader that can be configured to do this work for you on the upload. One example is the plupload module / widget which has a clientside resize option during the upload (also saving bandwidth)


resize

Enables plupload to resize the images to clientside to the 
specified width, height and quality. Set this to an object with those
parameters.