I think most of us realize over the years the material for the lines have changed to allow using smaller diameter line for the same strengh. However several things need to be considered before changing the size.
Some include other hardware
you have on the yacht. Clutches, self tailing
winches, stoppers, and even how it feels in the hand. Most have ranges of line size that may be adjusted. The only thing that runs better with a smaller diameter is blocks. Hate to spend $200 on the line and find you need to spend another $1000 on upgrading the hardware
it runs through. Also some of the newer high tech stuff allows to go very small but a 40 footer jib
sheet of 1/4" won't feel good in hand when the wind
Go to any of the cordage websites Yale, Newengland, Sampson etc. They give good advice on what will work.