1) population in USA: 1918 103m vs 2020 331m

2) Spanish flu death distribution by age was totally different (mainly "young" people were dying) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3734171/

So if we considere the "years lost" (life expency minus age of death multiplied by the number of cases), the spanish flue was way more impacting.