Turns out that when you tag a new image, Podman leaves the old image dangling instead of cleaning it up. Over time this means you have left-over images which, given the size of bootc images, accrues a lot of disk usage over time.
My fix for this was to add an additional post-run command to the build unit that prunes unused bootc images after apply, identified by the label containers.bootc=1.
[Service]
ExecStartPost=/usr/bin/podman image prune --force --filter=label=containers.bootc=1