One needs to be cautious about using only total expenditures for a number of reasons. First, purchasing power differs. For example, according to official statistics (which may understate military expenditures by Russia and China for obvious regions e.g., to mask support for strategic programs), China's and Russia's military expenditures are just under 50% of U.S. expenditures. However, on a PPP-basis, their spending is about 80% of U.S. spending. Second, allocation of spending also matters. The U.S. spends a higher share of its military budget on salaries and benefits than do either Russia or China. Third, inefficient practices e.g., the military bidding-contracting-delivery process can also reduce the benefits of spending. The Pentagon has had a chronic history of cost-overruns and delays in numerous big-ticket programs (e.g., the F-35). Data concerning such issues in Russia and China aren't public--almost certainly some inefficiencies exist but the extent is unknown. This inefficiency should be a focus of reform, as programs that ultimately produce weapons systems at twice or three times the cost originally set forth in accepted bids are wasteful and they undercut the power impact from military investments.
