Take a longer view – 250 years ago, population of Europe was way larger than USA. Then, for some reason, they/we all started moving here. So, something is done right in the American system in general. When comparing expenditures, you may want to
(1) review outcomes by different social/ethnic groups – USA is full of newcomers and groups of problematic population. Try comparing a person with origins in say UK or France n both countries
(2) make sure you compare all expenditures in European countries – cost of government policies gets diffused in many areas. Look for example at where medical research is being done.