Combined log format POST size
Since it's a POST, I presume that what you mean is that the user uploaded a 150KB image in this request?
That's not what's being logged -- the size in the log entry from %b (the last field in common log format, the default) is the size of the server's response to the client's request; in this case, the request might have been 150KB but the page the server sent in response, after the image was sent by the client, was 13KB.
If you want to log the amount of data sent by the client, enable mod_logio and put %I in your log format.