I'm not sure what you are thinking here.
If you make 100K, your take home pay after taxes is ~$6,000. For any reasonable place to live in SF - your going to be between 1800 and 2500 for a two bedroom.
Thats not "a small portion of your income"
Additionally - you may have health care costs of as much as 750 per month if you have a spouse and child on your plan.
Figure as much as 400 to 600 per month for a car. 100 for cable, 150 for an unlimited family cell plan. School costs for your kid, and god-forbid you. like me, have child support to an ex.
So:
6000 Income
750 insurance
2000 rent
100 internet/tv
150 cell
100 power
1000 kids school/child support
250 other debts
600 car+insurance
== 4950
That leaves 1050 for the remainder of the month. Which is ~260 each week for expenses such as food, gas, savings, a beer with friends, date night, a bottle of wine etc...
100k is literally the bare min in SF to live moderately comfortable.
Now assuming you didn't have the school/child support, you can replace that with savings, which is great. but for a lot of people it is a 1K expense not savings.
Additionally - this doesn't account for a lot of other potential monthly expenses that are common.