Right now, when displaying the course/report/log there is one function in charge of calculating the selector form. it's the print_mnet_log_selector_form() function.
One the fields in that selector form is list of users in the given course. For SITEID that list can cause memory problems, due to the number of records returned by the get_site_users() function (and potentially in courses by the get_course_users() ) function.
The selector form includes code to prevent the list of users to be rendered if there are more than COURSE_MAX_USERS_PER_DROPDOWN (1000) users but that doesn't really helps in big sites because the program breaks before checking that.