It's more than half, actually (IIRC upwards of 70%). Colma was specifically incorporated as a necropolis, and the dead outnumber the living by something like 1000-to-1.

Sure, maybe a bunch of cemetaries ain't the most efficient use of space, but it's certainly a really cool use of space. I live in close proximity to it, and appreciate it as part of the not-quite-green-belt between Daly City and South San Francisco.