Update: It just happened again with the same 2 hosts backing up to a new repo.
restic diff (latest, size is off vs. the previous one, where the size is normal) shows the files correctly (especially the large tar file being ‘replaced’ [old file -, new file +]). Size seems correct, too:
Added: 1.089 GiB
Removed: 1.031 GiB
Still, restic stats latest shows the snapshot size to be 28.435 MiB.