O formato do log nativo é:
"%9d.%03d %6d %s %s/%03d %d %s %s %s %s%s/%s %s"
Com a 5ª parte (% d depois% s /% 03d) sendo:
bytes The size is the amount of data delivered to the client. Mind that this does not constitute the net object size, as headers are also counted. Also, failed requests may deliver an error page, the size of which is also logged here.
ref: link