To be treated as "so much [X]" is an idiom, meaning they were treated as merely [X], or no better than [X]. In this case, the passengers in "steerage" class are being treated as just cargo, not as people.
The addition of "more" to the idiom, as in this case, is unique in my experience. It seems plain (to me, at least; others might disagree) that it means the passengers were just some more cargo, no different than the rest of the ship's cargo.
EDIT: In order to find examples, I did a Google search for "as so much dead", quotes included. I chose this search because I felt like I remembered the phrase "so much dead weight" as a particular example of the idiom. (Adding "as" to the front was necessary to filter out unrelated results.) Here are some notable results, with my paraphrasing:
...I am content to reckon my outlay on the book as so much dead loss...
"What I spent on the book is nothing but a loss (i.e. lost money)."
In the United States, the body is treated as so much dead matter...
"A dead body is treated merely like dead matter (not like a person, as in other cultures)."
...but when ascending from the lower to the higher, it acts only as so much dead weight.
"The component being described doesn't do anything but act as weight in this situation."
Using "like" instead of "as" gave me nothing but repetitions of a single song's lyrics. On the bright side, those lyrics are an example of the idiom!
Ain't it funny how/Life can drag behind us/Just like so much dead weight.
Using "just" instead (i.e. searching for "just so much dead") gave me even more examples. But please don't think that "dead" is a necessary part of the idiom! "Dead weight" is just a particularly likely thing to call something when you're saying it's no better than, well, extra weight.