Social Security Expansion Act This bill amends title II (Old Age, Survivors, and Disability Insurance) of the Social Security Act to: (1) increase the primary insurance amount for all eligible beneficiaries, beginning in 2023; (2) revise computation of cost-of-living adjustments to use the Consumer Price Index for Elderly Consumers; and (3) increase the special minimum primary insurance amount for lifetime low earners based on years in the workforce. This bill amends the Internal Revenue Code to: (1) apply employment and self-employment taxes to remuneration up to the contribution and benefit base and to remuneration in excess of $250,000, and (2) increase the tax rate on investment gain from 3.8% to 10% and allocate specified amounts of such tax revenue to the Federal Old-Age and Survivors Insurance Trust Fund and the Federal Disability Insurance Trust Fund.