That might help with extraction, but doesn't it increase the forces on the bolt face and locking lugs considerably when firing?
No, the bolt lugs and locking recesses are sized ignoring case friction. The case is not a structural element, it is a gas seal.
Assume you are designing the action. On a scale of 0 to 100%, just how much case friction do you assume, and thus by how much do you reduce the size of your locking lugs?. Do you assume 100% case friction, 50% case friction, 25% case friction? If you pick a number, other than zero, is that number within the control of the designer? If you assume any case friction, and cut your lug size accordingly, then the first time a drop of water, perspiration gets on the case, the action is now over stressed.
This is from Brassey’s book on Design Principles of Military Small Arms. Locking systems are designed ignoring case friction.
By the way, in the book, “The Great Rifle Controversy”, by Ezell, on page 219, early users of the M16 were ordered to oil all ammo and magazines to reduce failures from extraction. This was discontinued because oily ammo attracts dirt, but that is a field issue. Those who are humping their rifles through the boonies in a monsoon can’t keep themselves clean, never mind their weapons, but we who only go to the range and back home again can keep our weapons and ammo out of the dirt.