You are right… there hasn’t been much effort towards this… migrating to a newer OS was higher in our priority list and @hestia_hacker’s recent amazing work for Debian 9 (Stretch) left us a bit worried about how much more juice can we pull out of Pi Zero as it is not as responsive with Stretch as it is now with (ancient) Jessie.
Maybe newer OpenHAB will be heavier, maybe not. This is not something we have explored yet. Any help would be welcome