Sql Server Maintenance Plan - Simple Recovery Mode - Adding Shrink Log
The first thing you have to remember is that all transactions are logged to some degree.  There are cases where an action can be minimally logged but there is still some entry in the log.  

The recovery modes just change how often the space in the log is freed for re-use.  In all cases the log space is freed up once it is no longer needed.  In the case of SIMPLE recovery there is no backup required for the space to be freed.  However as long as a transaction is active that space won't be freed.  In your particular case you probably have some long running transactions (probably batch jobs) that are holding the space open and causing additional data to be added to the end of the log forcing it to grow.

If the logs are taking more space than you are comfortable with your best bet is going to be to examine any batch processes and try to shorten any transactions they are using.