The Death Star was completed in about 20 years, and as of Rogue One its construction was put on hold for a few years when the main designer went AWOL. The Death Star II was even larger and constructed to a functional state in a few years.

Although we've seen no advanced construction technology, the Star Wars galaxy more than makes up for it with sheer scale. The Kuat Shipyards, where most of the Imperial fleet is built, is an orbital scaffold that rings the planet, and it's large enough that its scaffold structure can be seen from the ground.

Star Wars' method of transportation is also superior to that of Halo, which means that manufacturing and resource exploitation can be spread out over entire star sectors, if not the galaxy itself.

: The Forerunner have hard light and
: Constructors. Things like the Death Stars and their most massive ships
: we're talking about orders of material larger than you'll find in a given
: system.

Then you're not looking hard enough. Once the asteroid belts are depleted, who will miss a few moons? Or even that rocky planet over there?

: But then again the "Star Wars is massive" trend
: continues to its ridiculous population and settlement stats. Wookiepeedia
: apparently gives a stat of 50 million inhabited systems. But perhaps this
: is more wrong with me, and stuff being all beyond comprehension given our
: current technical development (although then I'd say if you can do all
: that wondrous tech society would probably be unrecognizable to us as well,
: and then you get into the whole limits of sci-fi conversation.)

Which is why Star Wars is best classified as "Fantasy set in space". It's mythological and fantastical, but also familiar.

I think George Lucas's original plans for Star Wars would have been closer to what you imagine, but people who read his first draft told him that they couldn't make heads or tails of the jargon he was using.

: An interesting bit of head-canon. Fleet Battles and the like seems like the
: only thing that even remotely touches on that, though. Though I wouldn't
: put it past a Hierarch then you have to wonder how they are able to field
: that large a force without everyone else knowing. Just goes to show you we
: still sorely need more information about the exact political setup of the
: Covenant and its jurisdictions (said no one else besides me, probably.)