What options are you using with BCP? I found this in BOL

When bulk copying large data files into an instance of SQL Server, it is possible for the transaction log to fill before the bulk copy is complete, even if the row inserts are not logged, from the extent allocation logging. In this situation, enlarge the transaction log, allow it to grow automatically or perform the bulk copy using the -b or BATCHSIZE switch, and set the recovery model to simple. Because only committed transactions can be truncated, this option does not free up space during the bulk copy operation if the -b switch is not used; the entire operation is logged as a single transaction
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