Found this on books online:
Transactional Replication and the Transaction Log
If a database includes one or more transactional publications, the log is not truncated until all transactions relevant to the publications have been delivered to the distribution database. If the transaction log is growing too large, and the Log Reader Agent is running on a scheduled basis, consider shortening the interval between runs or setting it run in continuous mode. If it is set to run in continuous mode (the default), ensure that it is running.
Hopefully this helps someone later!