Considering the enlarger as a camera, I came to these assumptions:
For 1:1 reproduction, I agree with your 'reversed proportional' theory. i.e. distances and depth of field/depth of focus being equal.
When focused at greater distances, a huge change in subject distance translates to a small change in lens to film distance. e.g. a change from infinity focus to ten feet may result in a lens position change of about 1" (thinking about a view camera here).
My thinking then is that if you go in the opposite direction, e.g. the image on the film (or paper on the enlarger) is larger than the subject, then the reverse should be true. i.e. a small depth of field at the subject (negative) translates to a larger depth of focus at the paper.