As opposed to four engines. Losing number 1 out of four, offhand, would require a larger VS, than losing 1 out of two, because the moment arm is greater on the four engine airplane. But this is alleviated on some aircraft by reducing power, 50%, on number four. You still have more than half the total thrust left. On the twin, you cant reduce power on number 2, because you've already lost half. Hence the larger tail.
During statistical aircraft sizing, the engine out yawing moment is balanced by a certain historical value of vertical tail volume coefficient,
(Vv = lv Sv / b Sw)
Where Vv is the tail volume coefficient and will be in a statistical range.
lv is the distance from the cg to the a.c. of the tail
Sv is the area of the VS
b is wingspan
and Sw is wing area.
As the design progresses, and more information on engine thrust becomes available from engine development, the VS tail size becomes locked in to balancing that engine out moment, allowing for future growth.