We ran into a case this weekend, where watchdog grew to 116G+. While the exact cause still isn't known, it seems possible that an errant watchdog() call could of passed large arrays in the variables, and that these arrays were written to mongo.
I know the drupal way is to fix bad code, but given (a) watchdog is a logging/error detection tool, IMO it should do no harm, and (b) since the variables are simply passed to t() they should always be strings; I propose this patch to guard against bad code.