high mysql sent traffic
I have three conjectures on this situation. One or more of them may apply...
CONJECTURE #1
There may be other operations that happen apart from fetching data back-and-forth across the network. Here are a couple of of my old posts where I actually discuss Bytes_received and Bytes_sent

Apr 18, 2012 : MySQL : Does &#39;bytes_sent&#39; and &#39;bytes_received&#39; include mysqldump data?
Apr 13, 2014 : Determining replication traffic

In these posts, I talk about how MySQL Replication creates traffic in its framework. How ?
There are two threads involved for MySQL Replication

I/O Thread : TCP/IP Connection back to its Master read binlog events and record the binlog events in the Slave's Relay Logs.
SQL Thread : DB Connection that reads binlog events from the Relay Log and executes the SQL. If log-slave-updates and log-bin are enabled in the Slave, the binlog event whose SQL was executed in this thread is recorded in the Slave's binary logs.

If the WordPress DB Server is in Master/Slave Replication topology, note the effect

If DB Server is a Master, any Slave's I/O thread connected will make a Master's Bytes_sent increase.
If DB Server is a Slave, the I/O thread will increase Bytes_received reading binlog events from the Master and the SQL Thread will increase Bytes_received reading binlog events from the Relay Logs.

Based on the numbers you are showing, I would suspect the DB Server would be a Master and it replicates any log changes to one of more Slaves.
CONJECTURE #2
If the DB Server is not in MySQL Replication Topology at all, I would then suspect some exporting operations, such as mysqldump to another Server or even locally. Why ? A DB Conneciton is still require by mysqldump. Bytes being written to a dump have to be recorded in Bytes_sent.
CONJECTURE #3
If there are neither MySQL Replication nor regular mysqldump backups in place, then I can only see one thing running up Bytes_sent so high and leaving Bytes_received so low: DB Monitoring.
Someone asked this in ServerFault back in August 2011 : 1 billion mysql queries in 24 days? Can something be wrong?. In that post, I explained that running SHOW GLOBAL STATUS often can run up the status counts on Queries and Questions. Retrieving status values frequently can also contribute to increasing Bytes_sent.
UPDATE 2014-11-28 15:10 EST
If you control everything in Vultr and you know for sure that the connections are all from localhost, the problem may not be WordPress itself. It would have to be your either remote connections or just very frequent traffic spikes.
At this point, WordPress would be out of my wheelhouse. Try out the info in this post : How can I improve site/page performance of WordPress websites?