It's funny because from that diagram I really don't see any particular relationship between the shape and its content. You could draw a regular pyramid with three segments and write the same labels on it and it would make just as much sense to me.

If anything a regular pyramid makes more sense to me: you want the smallest/narrowest useful description at the top and then you gradually expand on it as you go down, providing more (wider) context and detail for the key information.

Edit: Of course, it's a widely used term and good to understand in that context; the Wikipedia link is useful.

I think it's about laying foundations at the beginning, not the length of the text at the beginning. The first sentence/paragraph is the foundation of everything beneath it, whereas the base of a normal pyramid is the foundation of everything above it.

> I really don't see any particular relationship between the shape and its content.

This is often the case with geometric metaphors. They catch on easily, but they rarely make a lot of sense on closer scrutiny.

Yeah, this seems to be true for most pyramid models. It's really annoying when you start to spot it.