I see this claimed all the time, but I don't understand why this is assumed to be the case.
GDP rises and falls with economic progress and innovation. If new LLM startups make a bunch of money, GDP goes up accordingly.
But government spending is primarily geared towards defense, infrastructure, and benefits/entitlements. What makes those costs go up proportionally to GDP? A fighter jet doesn't directly cost more because Amazon rolls out a new Alexa device, does it? America doesn't suddenly grow thousands of miles of new coastline because Google opens a data center in Iowa, right? If Rivian builds 10,000 new EV trucks, how much more interstate highway must be built?
I would expect government spending to ebb and flow as market prices fluctuate, and I get that they might be loosely correlated, but why is spending assumed to be so closely linked that we should expect it to be a function of GDP?
* Managing a large economy is more expensive than a small one: for example creating legislation and managing a justice system for 100 companies is cheaper than doing the same for 10000 companies.
* Tax revenue is typically more or less fixed percentage of GDP.
The first point is something you can't really avoid without impairing the government. The second one you probably could with political decisions.
Just a side note, but government spending change makes more sense when measures against GDP, not just inflation adjusted raw numbers.