Once you've executed on enough of these contracts, you develop an idea of what the unknown-unknowns are and plan for them.

It's government work, which automatically means access issues, dozens (or hundreds) of stakeholders requesting meetings, internal politicking, back-and-forth over change requests, etc. All of that costs real money and if you don't plan for it or make contingencies, you're going to be screwed.

These issues are not just limited to government either; any sufficiently large entity will have these same problems. So yeah, one person-year is a reasonable minimum viable contract period unless you have a process in place to fast-track RFP approval.

We did tons of tiny contracts when I was working in government contracting (DOD/DOE).

Okay?

So my experience (government contracts have a floor of one person-year) doesn't match my experience with government contracts.

I heard that relatively smart govs stopped doing stuff like this that the outsource whole projects, but now they "borrow" programmers e.g "give me 20 programmers with higher edu and 5 years of experience" and we will lead this project.

but I'm not sure how's the reality.

Sounds like a good way to get unmotivated programmers who sit around drinking coffee all day and not doing any work, because hey - the longer this project takes, the more bonus our employer gives us!

What makes you think so?