We also noticed an over sized NIS+ transaction log. We had hundreds of entries. It was in need of a checkpoint. Normally, if you have more than 3 entries, a checkpoint is needed.
Here is an example of a run we did.
# nisping -Ca
Then we checked for the number of entries in the NIS+ transaction log.
# nislog | head -15
The NIS+ trans log contained 3 entries; therefore, it was fully checkpointed.