This is the same with Albumen prints as well. You use tissue to reduce exposure and increase the contrast (I went on a course along with another APUG member this summer, the tutor is one of Mark Osterman's former wet plate students - small world)

It must have something to do with the self masking effects of the emulsion during exposue and the threshold inertia needed, so the fast that mask builds up the lower the contrast. So yes akin to reciprocity.

It happens with colour papers, the owner of my local Pro lab was an ex Durst technician and once told me that on the old roll head printers the lenses were set to give identical exposures regardless of print size (enlargement is done by using different focal lenght lenses rather than distance), this prevent contrast shifts and also colour shifts due to reciprocity.

Ian