How come rust built up on iron things looks like a thick crust, but somehow when it's removed it's like there's nothing gone from the original?

Because the rust is very fluffy, while the iron is very dense. So a very small volume of iron can turn into a large volume of rust.

The iron is disappearing though. Just slowly. You could tell if you weighed it with a sensitive scale.

often there is stuff gone, called pitting

Full disclosure, I’m not an expert in any scientific field, but I did have to read up on this in school one.

The gist of it is that iron is a solid at most temperatures found on earth, and other materials don’t penetrate it easily. Rust is formed by a chemical reaction of oxygen and iron, where the oxygen binds with the iron.

Rust is also a solid at most temperature found on earth though, meaning when enough rust coats the surface of an iron object, much like paint, the oxygen cannot easily reach the iron underneath, leaving it unchanged.

Over time, things could come along and cause that rust to erode away, which would then lead to more of the iron being exposed and more rust forming, but that takes time. Thus with larger iron objects, you’ll still typically find iron underneath.