Figure alone can be misleading. Need to take effectiveness and efficiency into consideration also.
The US is able to spend their defense dollars more efficiently then China. The US generally has more advanced military technology so each additional dollar spent is able to purchase weapons that can produce more damage.
Also the US has been spending vast amounts of money on defense for much longer then China has so it already has stockpiles of weaponry and technology to use if necessary
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China don't need to spend that much on defense. Attacking China would be a costly war and that would be the mutually assured destruction of the world economy for at least a decade, therefore the attackers would shot themselves in the leg.
(Not to mention that Russia has close ties to China so they would likely to be pissed off.)
At this point China only needs to build up technologically, keep strengthening it's own economy and to establish the means of their power projection for the future. The USA will bleed out economically within a few decades and will have to introduce some new, more introverted policies for sorting out it's own problems. That will be time when China can start spreading her influence over the word without getting involved in costly clashes with other power centers.