You are probably not backing up the log or truncating it on the reporting database. Everytime you add data, you're adding to the log.
If this isn't critical data and you can stand to lose some of it, then I'd set the database to "simple" mode in the options. This will truncate the log every few minutes and it shouldn't grow very large.
Not sure of how large your loads are, but the largest loads will determine the size of your log. You can always setup a job on off hours to shrink it if need be.