However big it makes sense to be?

I don’t get how “big” became a persuasive argument when it comes to the public sector, rather than “ineffective” or “inefficient”. Raw “size” isn’t a good target on its own.

At the end of the day all I care about is that things are done effectively and efficiently, and sometimes maybe we trade those things off so that they can be provided fairly and equitably (such as fire departments, I don’t care if a private sector solution would be more cost effective, I don’t want private companies selectively putting out fires).

That is a meaningless metric on its own. What matters is how much money you have left to spend on things you want to as part of a healthy, happy life.

In the US a lot of things are inefficiently pushed into the private sector, health care being a good example. Taxes + private health care costs in the US are a higher percentage of GDP in the US than in any developed nation yet the results are the worst of the same cohort (OECD).

(This isn’t to say that there aren’t things done inefficiently in government that might be better in the private sector. Just saying that single figures are worthless alone).

39.41% according to your own source.

It was ~32% pre-pandemic, and excepting for the Great Recession, looked to be stable at ~30%. Considering the governments revenue over that same period, again according to your own source, is also ~30% we're at less than 1/3 of GDP expenditure.

I think that is a reasonable public expenditure for a representative government, especially for the third most populous country on the planet and the largest GDP.

Paradoxically, if the government did more it would spend less. All civil servants without in-house expertise know how to do is RFP and spend money.