What is your interval for ingesting ~7.5M? Are you writing mutations or bulk ingesting? Assuming you are writing mutations, those writeahead logs will all be replicated 3 times by default in HDFS, so ~22.5M. Then it gets flushed to disk as some point, also replicated 3 times by default for another ~22.5M.
So a couple more questions.
1 - Do you have HDFS Trash enabled? While I consider that a best practice, it will keep the data around longer. 2 - Is all the HDFS storage in /accumulo?