The smaller rudder/fin versus bigger rudder/fin is something worth looking into on the 757-300. What is the engine duo fitted to this? I believe it's an uprated version of the RB211-535E4 as well as an uprated version of the PW2040.
The SP required the rudder increase for many reasons like those you mentioned, aerodynamically it fit perfectly because of the reduced fuselage length and the engine power was somewhat different on these models from standard -100/-200 models, generally in the 47000-to-51000 lb range, whereas most -100/200 models at that time (1976) were in the 43-45000 lb class. Later on the higher engine thrust were retrofitted or built onto new-build -100/-200s.
Another example is the new rudder/fin on the 737NG, same airplane in size compared to the 737-500 and 300, slightly longer than the -400, yet they opted for a bigger rudder/fin assembly. Better control? Wonder why.
Same holds true for the A340-500 and -600. It's going to be a more powerful and heavier airplane of course. But..the airplanes are longer than the A340-300. They will incorporate the A330-200's fin/rudder. By comparison, The A330-200 is less than a foot smaller in length compared to the A340-200, but it's power package of two CF680, PW4000 or RR Trent outstrips the 4 CFM engines on the A340 , and quite possibly something must have borne out in the design showing the benefits of a larger tail fin/rudder assembly on these types of airplanes.
I've wondered if there is a problem like this on Boeing's 777-300. What must compensate is it's fuselage structure being wider and 'beefier' therefore it may not "wobble", but the narrower a fuselage the twisting movement get's amplified. That's the "flip side to the coin"; fuselage width. The wider the fuselage, generally the twisting movement gets reduced.