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.
How do you draw the conclusion you like to draw if you miss these?