MySQL relay log rotates too soon and too often
You can verify by yourself as slave server creates (rotates) a new relay log file under the following conditions only:


Each time the I/O thread starts.
When the logs are flushed; for example, with FLUSH LOGS or mysqladmin flush-logs
When the size of the current relay log file becomes “too large,” determined as follows:

If the value of max_relay_log_size is greater than 0, that is the maximum relay log file size.

If the value of max_relay_log_size is 0, max_binlog_size determines the maximum relay log file size. 

The SQL thread automatically deletes each relay log file as soon as it has executed all events in the file and no longer needs it. There is no explicit mechanism for deleting relay logs because the SQL thread takes care of doing so. However, FLUSH LOGS rotates relay logs, which influences when the SQL thread deletes them.