The other part of the equation is that apart from the engine many 3/4 ton trucks are better equipped, frame, brakes, shocks, heavy duty transmission, to deal with the riggors of heavy towing. My own experience with towing my previous '01 27' Safari with a 1/2 ton van and and 3/4 ton van showed a remarkable difference in suspension. Most noticable was dealing with dips in the road. You know those nasty dips where the trailer goes nose down then pitches back up pulling the back end of the tow vehicle up, then back down again. Nasty stuff.
White knuckle when I pulled with the 1/2 ton, just a small dip and then back in control with the 3/4 ton. Difference? Bigger springs and shocks. Now you can get into a case of overkill and too big a vehicle carries its own issues like transmitting road shock back to the trailer which is no good either. So while bigger is sometimes better, its all relative to what you are towning.
Interestingly enough, it wasn't that I was unhappy with the performace of my half ton van. I just didn't realize how much better a 3/4 ton vehicle would make it.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'14 Honda CTX 700