A larger femoral head size has been shown to reduce hip dislocations and minimize instability. There are several reasons why they work. First, they provide a larger surface area in contact with the acetabular cup (socket).
Second, a larger head gives a better ratio of size from the head of the femur to the neck. That’s important in allowing a larger arc of motion and preventing impingement or pinching of the femoral neck against the rim of the cup.
The surgeon usually puts a polyethylene (plastic) liner inside the cup (between the cup and the femoral head). With this liner in place, the larger head can sit deeper inside the cup, making it more difficult for the head to jump out of the cup and dislocate. Studies have verified that a thinner liner can hold up while still accommodating the larger femoral head.
And improved materials have extended the wear on these heads. That’s important because larger heads do tend to wear more potentially reducing their long-term survival. Finally, larger heads can potentially eliminate the risk of early dislocation with implants of all kinds (e.g., ceramic, metal-on-metal, polyethylene). That makes them useful for a large variety of patients who need a hip replacement or revision surgery such as your grandfather requires.