"That much more so"

Question

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?

Answer 1

It's the meaning of that given as no. 11 here: "to a great extent or degree; very".

"That much more so" = "Very much more so".

Answer 2

make a [something thing] that much more so.

make an already sweet cake that much more so.

It means: that much sweeter.

Whatever something already is, it will be even more that way: the idiom is: that much more so, where the so refers to the state or quality of the thing being intensified.

The expression has to be taken as a whole like this: that much more [sweet, dangerous, so].

after the more, you can have a noun, an adjective, or so (used as a deictic reference

• that much more fun
• that much funnier
• that much more so. [functions to replace the adjective]
• that much sadder.
• that much easier.

X makes a Y that much more [Y]

It is not an idiom per se. It is, however, an idiomatic way of expressing the fact that some characteristic makes something that is already Y, even more Y.

• To make an already combustible situation even more combustible (or even more so).

has the same semantic meaning as:

• To make an already combustible situation that much more combustible.

• To make a pathetic movie even more pathetic (or even more so).

has the same semantic meaning as:

• To make a pathetic movie that much more so.

These are comparative adjectival clauses that describe the adjective + noun phrase.