When I first arrived at the Castle settlement, the workshop size bar was already at 80% of being full and I haven't even built anything yet? Is this a bug? I noticed that the size bar is never empty at any of the settlements the I discovered so far.

The reason that I asked this question is because I get a warning on the Pip Boy that the defenses at castle are at 75%, so I can't build any more defensive items to raise this stat

Is there official documentation where all of the workshop functions are explained in detail because the online help is really just too basic for such a complex system.

Stuff you can scrap counts towards it.

Now sure what you mean?

Before I arrived at the Castle settlement, the size bar was about 75 -80 % before I even started scrapping anything. I scrapped everything that I could find, but the size bar didn't shrink much.

From my experience with the size bar, it seems to be a set limit on how many items you can build per settlement because in the Castle as I started building stuff, the size bar got full and I couldn't build anymore stull.

I know if I scrap stuff the size bar gets smaller, but why was it at 75-80% before I even started building stuff in the Castle settlement?

There's a lot of electrical and decorative stuff in there as well as the stuff in the armory. Maybe that's boosting the count? Or maybe it's including some static objects as well? The point of the size limit is to make sure that the load does't crash you machine.

I scrapped everything including the armory before building anything.

yeah, the castle is rigged with lights, switches, a water purifier, the radio tower, all it needs is a couple of generators and some defence and it's good to go. You really don't have to "build" in it (unless you want to)