Why is the DC10/MD11/TriStar vertical stabilizer noticeably smaller than other contemporary aircraft?
Two characteristics drive vertical tail size in airliners:


Critical engine out: Here the tail needs to compensate the yawing moment of the live engine on one wing.
Yaw damping: This is aided by a yaw damper, but enough damping must remain for the case of a failed damper. Especially the dutch roll eigenmode is driving tail size.


On the other hand the tail should not be bigger than absolutely necessary to keep drag and weight down. On the Tristar, the big intake added so much tail area that a relatively small tail was sufficient. Note that the engine intake on the Tristar is much longer than on the DC-10 because of the S-duct. The flow must be brought down to the engine without internal flow separation, which is made easier when the intake is longer.

Since in the engine-out case only one third of total thrust creates the yawing moment which the tail needs to overcome compared to one half of the thrust in case of the A310, the vertical tails of both the Tristar and DC-10 are smaller relative to that of the A310. The A310 is a shortened version of the A300, so the reduction in fuselage length made an especially large vertical tail necessary. If you look at the vertical tails of the MD-80, the Il-62 or the VC10, they are even smaller since very little yawing moment needs to be compensated in the engine-out case. Adding to this, the T-tail configuration makes the existing tail more effective due to the endplate effect (PDF!) of the horizontal tail on those three types.



Vickers VC10 (left) and Tristar (right) tankers in flight (picture source).