People used to be thinner when these things were first designed.

It's only a plate carrier by the looks of it. No surrounding soft armour

It's called a plate carrier, as the name implies it carries just a bullet proof plate that covers lungs and heart from the front and back. They are very useful if you have a lot of gear you need to also bring flash bangs, IFAC, ammunition etc. Sometimes they don't even have the plate in them.