Your transaction log is growing to accommodate the large number of transactions, as it's supposed to. Frequent shrinking will cause fragmentation and as the log file grows again it will use more server resources to do so - both not good. To control the size of the log, schedule more frequent transaction log backups. Log backups free up space within the log for reuse so if you monitor this you should be able to settle on the optimum size for the log and frequency of log backups. Check out 'Using Transaction Log Backups ' in BOL.
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