"That claim is false: the overwhelming majority of US government spending is proportional to the number of people - and in particular, a significant chunk of it is proportional to the number of old US citizens."
"Much of the rest pays for equal-chance education (teachers), unemployment insurance (which cost goes up during crises and goes down during booms), poor families/children, roads and other infrastructure - none of those have fixed maintenance costs but go up linearly with the number of people."
All spending is proportional, but some groups are more proportional than others?
The rest of your argument is nonsense too. If spending were linear, add 33% to to 1980s spending (1/3 more people) and track it to inflation. You'll get a lot smaller number than today's spending levels. Hint: $3.4 Tril - 2.6 Trillion less than last year's spending. All you've done is just prove that the government is overspending by nearly double.
We wouldn't need more taxes if we hadn't increased defense spending to huge amounts. You'll get no argument from me about cuts there. Medicare and Social Security are, while noble ideas, also unsustainable. Both are ponzi schemes with most people putting in less than they get out - in which case the first out get the benefits while those that stay in require a larger and larger burden. We are fast approaching the point when the number of recipients are larger than the number of payors, the only way to counter that is to make payors pay more (tax them, tax the rich ones more (even though they don't need medicare/SS and arn't the problem)) or to increase birth rates and/or immigration to have a larger economic base in which to pay for the next generation - basically creating economic slaves out of our children to pay for our own retirement. Bush actually had a good plan to help fix the problem - 4 cents on the dollar invested into individual accounts- but we know how that went.
We (as in the government) have also added numerous agriculture subsidies, dumped money into various new departments and bureacracies, given raises and benefit increases to federal workers - and then the federal unions have forced contractors to pay their workers federal wage rate, (you should see what the construction workers on federal projects get paid!) and paid for hundreds/thousands of pork-barrel projects (war on drugs, bridges to nowhere). If, instead of allowing special interests to mine out the money of the people in every way imaginable, we had simply focused on infrastructure, education, and modest social support, and modest defense spending, we'd be just fine.