Arms race is not a one to one spending contest. One only engage in arms race in the theater that one seeks to doiminate, while making sure one is reasonably protected against the opponent’s retaliation.
China is still far from being a global military power, and China is only seeking to dominate the west pacific in the narrow sense. The US is a global power, and seeks to maintain dominance in the west pacific in the broader sense, as well as the Indian Ocean, the Mediterranean, and North Atlantic, with sufficient reserves to deploy to establish dominance in any of the world’s other parts on short notice.
So for the time being China only needs to outspend the part of US defense spending that could be brought to bear in the narrow west pacific without jeopardizing US dominance elsewhere.
So the % GDP comparison is worthless. China is engaged in an arms race with the US, And China is astutely leveraging the fact that it is still a rising power that has as yet accumulated relatively little commitments outside its own immediate region, and exploiting the fact that as the Long established global power, the US is overstretched with many critical commitments besides its hitherto peaceful contest with China, and much of america’s Military spending can not be brought into play in its contest with China. The last thing China should do is to expand and diversify its global interests too quickly that its interests overlap with many of america’s Interests around the world, and much of americ’s arms investments for other parts of the world would automatically be brought into play.
While USSR had a weak economy and limited real global interests outside of its immediate environs st the end of WWII, Stalin sought to engage in comprehensive arms rave with the US, seeking to gain parity with the US and contest US supremacy in every theater the USSR can access. This was stalin’s Big geopolitical mistake. For ideological and internal political reasons the USSR could never back away from that overambitious position after stalin’s Death without making it look like world communism is losing ground to the USA. Sticking to a comprehensive arms race that required comparable total military spending eventually brought down the USSR.
Putin, astute as he might be, appears to have, whether by design or accident, landed himself and Russia in the same trap again. His position require the commitment of a larger percentage of a smaller resource pool than his opponent.
China, on the other hand, has done a better job, and have hitherto comported itself such that in its disagreement with the world’s premier power, it needs to commit a smaller percentage of its own resource pool than its opponent.