To get an idea of shipbuilding productivity it would maybe be more useful to compare the number of people employed in shipbuilding rather than the population of the entire country.
(yes, of course total population is one limit on how many ships you could build in theory, but societies tend to have many other priorities than building ships.)
The idea is what level of ship building is possible if a country prioritizes it by building the relevant workforce and industrial policy. Cursory research suggest US and SKR both ~120k employment... which kind of makes disparity even more ridiculous. Almost unbelievable, I'll have to check numbers again when I have time.
Otherwise my understanding is US shipbuilding incompetence is labour - attrition rate at 20-30%, can't develop mature workforce if turnover every 3 years. Because work conditions shit and pay comparable to fast food and retail. IIRC result is a ddg that should take 6 years to build is taking 9 because constant retraining and reworking. There's actually plenty of orders / backlog, but the workforce conditions sucks.
(yes, of course total population is one limit on how many ships you could build in theory, but societies tend to have many other priorities than building ships.)