On reason why US welfare expenditure is so high is surely down to the incredible inefficiency (form a financial point of view) of the American medical system. It's not commonly appreciated that the US government spends almost the same % of GDP on their public systems (Medicare and Medicaid) as the UK government does on the NHS. Given the difference in coverage, this is astonishing. It's around the 8% mark in both cases, and in the same general area as many large western countries. It can't even be explained because the US has an older population. It doesn't, but rather the reverse.
A lot of this must come down to the basic cost structure of the US medical industry with al the insurance, legal indemnity, billing and other cost issues (and some very well paid medical staff).