So Sanders isn't suggesting randomly hiring 13 million people - he's suggesting that the country's infrastructure needs serious investment in order to remain competitive. This is a mainstream opinion held by almost every major analysis of the US infrastructure.
To me this is huge. Our country's infrastructure is literally falling apart.
That is mostly due to decades of misappropriation of highway tax funds at the state & local levels. Simply spending more on major infrastructure projects without dealing with the bidding system will lead to more graft, not necessarily improved infrastructure.
Exactly. There are few people who think we don't need improved infrastructure. The problem is trusting the government to do it efficiently and without graft.
You say that as if the government doesn't hire it out to private contractors already. If there's inefficiency and graft, it's either in the procurement process (which "small government" wouldn't really improve) or in the private industry itself.
No, the
real problem -- and I say this as an engineer who would tend be biased in favor of more infrastructure -- is that a lot of the infrastructure we've built in the past 50 years has been a gigantic mistake. It was never sustainable and was never going to become sustainable; we just apparently didn't realize it at the time.
The trouble is that when we allow suburban sprawl, we increase the amount of transportation infrastructure we need not proportionally to population, but
beyond proportionally to it. When everybody lives in a city ("point A") it's easy for them to get where they need to go. When there are a few suburbs (points B, C, and D) and everybody commutes into the city then it's still relatively easy to accommodate their needs by building highways in a star topology.

But as the city and suburbs get bigger and traffic gets worse, jobs move out to the suburbs too -- and that's a disaster: instead of having everybody commute to A, you have people commuting from B to C or from C to D or any permutation of locations, and to get them there in any reasonable time at all means you have to build lots more roads in a topology that looks more like a mesh.

Because of this principle, there will be a point where we simply can't afford to expand the number of suburbs and will have to accommodate increasing population (assuming population continues to increase, which is not as certain as it used to be) by increasing density of the existing urban areas and switching to more efficient (i.e., cheaper per person, assuming full utilization) things like rail transit.
Not to mention, all of this stuff has to be maintained, and as the size of the road network increases, so does the ratio of "existing" roads to "new" roads. It's no surprise that at some point maintenance of the existing network approaches 100% of the budget (or exceeds it, if you've planned poorly by deferring maintenance before); it's mathematically inevitable unless the budget is growing at a faster rate, and it's not. (Because it
can't.)