You normally have the entire extractor groove exposed, plus enough of the case above that for the top of the extractor. Sometimes the barrel is brought back to the extractor groove and cut away for the extractor hook to rotate, like in a standard AK barrel.
Various designs also have reliefs cut into the bolt for the ejector. These usually don't compromise strength at the breach.
Some modern designs use a very narrow extractor hook, usually coupled with a spring-loaded in ejector built into the bolt face. The front of the bolt will socket into the back of the barrel. That's about as good as it gets.
Designs like the .30-06 Springfield and M1911 Colt didn't have particularly good support around the case head, so (since they were designing the ammo too) they made the case head thicker to handle the load.
My MAC-11 fires from an open bolt. The "firing pin" is a nub about .040 high. Almost by definition, it fires out of battery. It's not uncommon for it to blow the side of a case out ahead of the extraction groove, or eject pretty little fireformed cases that would fit nicely into a .380 Belted Magnum, if such a thing existed.