You'd be surprised how much shock force you can put down with both arms locked, both hands focused into as small an area as possible, your full body thrusting on top, adrenaline flowing, and a person under you who is already injured or has chronic health issues.

If you break a rib though, generally speaking either they already had broken ribs or you weren't doing the correct compression technique for CPR. (Edit: or chronic health issue thing ofc)

Source: I am a CPR-certified first responder