Also, California has realigned a number of what were previously (and are still in other states) state functions to counties, including some felony incarceration that would otherwise be in state prisons, so, even compared to out-of-state city and county combined functions, SF has more it is required to do.
Seattle's population is around 755,000. 755,000 / 2,400,000 ≈ 0.315. 10,200,000,000 * 0.315 = 3,213,000,000.
3.2 billion + 8.3 billion = 11.5 billion.
15 billion (San Francisco budget) - 11.5 billion (Seattle budget) = 3.5 billion.
So San Francisco's budget is around 3.5 billion dollars higher.
[1] - https://kingcounty.gov/en/dept/executive/governance-leadersh...
Also, sound transit provides much transportation services in Seattle, but it is not part of city of Seattle. On the other hand, SFMTA is a department of city and county of San Francisco.