Two lines in the movie The Kingdom go:
And after speaking with Thamer, I advised withholding additional U.S. Personnel, because a large part of the religious justification for these bombs is the presence of current U.S. Personnel. More boots on Saudi soil is only gonna make an already combustible situation that much more so.
I am not sure what "that" does in this sentence. Without a point of reference, "that much more" doesn't make sense to me. "That much more"--how much more exactly? Why is it "that much more so", instead of much more so?